Lay the plate on the top and use a fine marker to draw the outline. remove the plate and draw a second line half an inch inside the first. Cut the opening with a jig saw to the size of the inside line. Lay the plate to fit the original, outside line. Lay boards out as illustrated around the plate (pix 1). Fold a couple of playing cards between the boards and the plate so it will be loose enough that you can get the plate out more easily. Clamp the boards in place, remove the plate (Pix 2). Use a mortising bit (Pix 3) with a bottom mounted bearing (on the shaft, not the tip) and set the depth just a little deeper than the thickness of the plate. Use the clamped boards as your guide and cut a lip for the plate to rest on.
You could drill small holes in the rebate and use screws up throught bottom to level the plate with the top, or take the easy way out and buy some Kreg levelers a(Pix 4).
Doesn't take that long to do and you don't need any special tools to do it. I choose a Woodpecker aluminum plate for my 3.25 hp table router (Pix 6). That plate is pricey, but it is extra thick, and has a twist lock setup for the inserts. No tiny screws to mess with and misplace. I don't bolt my plate down, the router's weight is sufficient.
You might want to get a mortising bit that matches the radius of the corners of the plate. The alternative for the Woodpecker plate was to purchase their mdf template, with will produce the proper corner curve. I think other plate makers also offer templates. You will likely only use it once.
The more I do, the less I accomplish.
Last edited by DesertRatTom; 09-08-2019 at 10:51 PM.