Levelling a home made Insert Plate

I prefer a two layer table, MDF on the bottom, VERY VERY flat ply on top, carefully glued together along with screws. Cut the full opening in the top sheet (use half inch Baltic Birch if you can find it). Cut a smaller opening in the bottom layer to form a lip, then pre-drill openings for your leveling screws to go in. You could also use threaded inserts, but not necessary.

The other leveling option is easier. Order a set of Kreg leveling screws that fit in the corner of your opening, about $20 a set of 4. You can do this with a single layer table as well, but If you can, add that second layer, it will hold screws better over time. Wax and polish the top with a powered buffer, not by hand. Here's a pix of the Kreg leveling screws. It shows a rounded corner but will work as well with a sharp, 90 degree corner.

Many folks would prefer a laminate top. If you do that, you'll want to roll on contact cement as smooth as you can, let it dry then use dowels or sticks to hold the top and laminate apart. Once they touch, they are glued forever. You pull dowels or sticks out one at a time, press that section down with a roller, starting from the center, out to avoid trapping air bubbles. Remove another dowel, roll from the already flat outward and center to the edge. Once down flat, you'll use a trim bit to cut off the excess. If you trapped an air bubble, use a small drill bit with a stop so it penetrates the base, but not the laminate. Measure carefully to locate the center of the air bubble, drill and let the air out, then roll it flat. Cut your openings before you apply the laminate.

But ample, polished wax will do.

The weight of the router should hold the plate down just fine. The leveling screw comes up from underneath. Graeme is correct on that. With the Kreg device you don't need a rebate, although a rebate can help keep the router and plate from falling through while you remove it to make adjustments.

You can use the baseplate for your router to mark the locations of the small bolts that attach the router. Use a marker to circle the bold locations before you remove them. Don't lose the bolts! Use a punch that fits snugly in the baseplate's marked openings to locate where you will drill. Drill with a drill press if you can, or at least use some sort of drill guide to make sure you drill straight. Drill a larger hole a bit larger than the diameter of a flat topped screw. Then drill a smaller hole in the center of that, but allow a mm or so extra so you can allow for any drilling error. The bold in the illustration doesn't have threads on it, but your small bolts will be threaded.

BEFORE YOU MARK AND DRILL:
Trial fit your baseplate in the table. Any adjustment needed?
Rotate your router so the controls will be oriented toward you when using the table, particularly if it has a lock for height, or a detent button to stop the collet nut from rotating when you change bits. Once you have that worked out, you'll know how to orient the base on the plate.

Notice in the drawing that the bolt is not centered precisely in the opening, that represents the allowance for a drilling error.

Do I rebate slightly over depth then drill and tap the four corners of the plate to take an adjusting screw of some type? also if that is the way to do it I'm guessing screwing the screw onto the mdf itself wouldn't work very well so do I fix a metal plate etc. in each corner of the rebate for the screw to sit on?

Would the plate also need fixing down to the table or will the weight hold it in place?

Greetings, from a land where Kreg levelers are available, but burn holes in the bank balance.
1. I have a plate that came ready-drilled and tapped, with small grub screws, for leveling purposes. Since I used a laminated 32mm countertop for the table, with a rabbeted lip, I drilled a hole in each corner of the lip with a
Forster bit, and epoxied a small rare-earth magnet in each. The grub screws have a much more durable surface than the exposed chipboard against which to bear, and there is some additional down-force on the plate. Also, the grub screws are less likely to rotate, and change the height adjustment.
2. Whether any other fixation of the plate is required, may depend on the weight of your router. In my first table, I was using a powerful but lightweight Hitachi router, and had the uncomfortable experience of the router lifting the plate out of its recess (and even lifting the fence enough for the plate to twist about 20 degrees), when the bit hit a knot, or something. Since then, I have tended to be a belt, braces and piece of string kind of person. My current two table-mounted routers both weigh enough that I feel comfortable they will not dislodge, no matter what. But the magnets give some additional (placebo) comfort.
BTW, heavy routers would require either a double layer of Mdf or whatever, as described by Desert Rat Tom, or the type of underpinning used by Joat, otherwise over time the top will sag.

Dan, I plead no contest. However, when the choice is between mdf, chipboard, or construction -grade plywood (shuttering) on a limited budget in a place where imported stuff is expensive, we have to resort to what DRT calls “Third World ingenuity”. The mdf is locally produced (and exported), and is generally dimensionally very accurate.
Given that I have no desire to exceed three-score and ten, mdf will probably outlive me. The current two router tables have given me many many years, without problems.
Understand that around here, Oregon pine was originally imported (from the US), and now serves for trendy woodwork (in recent decades, there have been a few small local plantations ofDouglas fir).
In fact, I have just made a shelf unit for my consulting room out of recycled DF floorboards, stained to not clash (note I did not say match) with my real partridgewood desk - an beautiful indigenous wood from tropical countries to the north, but entire forests have been sold illegally to the Far East, none left.
The problem here is that Northern trees, when grown here, grow too rapidly, so the wood is spongy (yellow pine is harvested at about 20 yrs, construction use only). The indigenous good woods are slow-growing and scarce - they were over-harvested in the old days by the colonists, often for trivial use.
One can source imported quality plywood, even marine plywood ,but the price hurts. So, melamine-faced man made boards are the order of the day. I envy our American friends the price of their 110 volt power tools, and I envy the northern hemisphere in general for the choice of affordable hardwoods.

Actually pictures work better. The allen head set screws are above the insert at this point as I haven't leveled the insert with the table yet but that also helps in what I'm describing. In this case there are 10 leveling screws (silver). 3 front and back and 2 on each side.

This was my latest solution. Not very elegant but just as functional as any other method and works very well with mdf. Only took a couple of minutes to get my plate level.