Hi, I had some "fun" trying to make the sled runners from hardwood, seems like the only way to properly make them is to have a planer/jointer or to make them from plywood/MDF.
I wasted some hardwood trying to make them, with no success, the wood was not perfect enough to allow me to use my fence, however, MDF is perfectly flat and in 2 mins I could make two perfect fit runners.
However, I fear that MDF is not a good material for this job.
I've made quite a few cross-cut sleds... and I've tried a variety of materials for runners. I'm assuming you are taking about 3/4"x3/8" Miter slots or 5/8"x1/4" slots?
MDF is good left being used as interior panels, carcasses for cabinets, millwork, etc.. If you cut then into thin strips to use as runners, they tend to want to separate and split. When MDF is made, it is pressed together form the sides... and when stress is relieved, as when you cut into thin strips, it wants to come apart in thin paper-like layers. Even if it does stay together, it is not very durable and wears out fast.
I've tried UHMWPE... When cut into strips it wants to flex and bend. When I used that, I cut grooves into the bottom of the sled to mount the miter bars into, to keep them straight. To tell you about the experience, I didn't make any more like it. I have UHMW on me fences and I really like it there, but that is in slots in extrusions, so not the same kind of affair.
Once upon a time (years ago) I thought about plywood... but when you cut plywood into strips, relieving the stress in it, it has a mind of it's own. Very few strips will be even close to being straight. And iif it is off by a smige, a PITA to get down that small amount.
Most of the sleds I still make, I use hardwood or metal bar stock. I've had really good luck with metal key stock... It is durable and I can use nylon set scews to make them adjustable in slots. The metal miter bars are handy, because I can drill and tap the bar stock to screw the sled to the bars. (very strong that way)
But hardwood is cheaper and more readily available. I've had the best luck using scrap pieces of oak and maple hardwood flooring. The best one's I have were made from apitong. (but I had some scrap onhand = too spendy to go out to use just for that...)
Easier with a planer... but not required. Look in my uploads for pic's of one on my TS jointer jigs. I make them in 4' and 8' lengths. I use them as a straight line guide for Circular Saws and a glueup ripping, as a jointer guide or to get a straight cut on rough cut lumber... on my cabinet saw or on my jobsite saws.
Easy to make- A 4 foot long piece of ply wood and a 4 foot long piece of 1x2. Screw the 1x2 to the plywood's factory edge. Take your circular saw, with the wide edge against the edge of the 1x2 and trim the plywood.
You now have a guide with 2 parallel straight edges. If you put a board on top of that with the bow away from the 1x2, the guide with the 1x3 against your fence. You can get a "straight" quality glue edge rip (on your table saw).
I use another strip of plywood on top of this guide with a few toggle clamps to hold strips of flooring to cut strips for miter slots. It's just a lot safer to do it that way. You never have to have your hands or fingers near the blade... and it comes out with very nice cuts.
Glue-edge cuts on dense hardwood, I usually use a 10' or 12" glue-edge rip blade, 40 tooth. With dense hard woods, I can sand them to adjust the fit. Then I wax them. I make enough sleds (often enough) that when I have hardwood scraps that look like they would make good sled miter bars, I put them on a hself with my other alike stock. When I get enough of that, I set up my saw and cut a number of them (using the same setup)... That way, I have them on-hand later when I get in the mood to build some new sleds.
I've got a planer and planing jigs, but to tell the truth, I've never used them for making miter bars. On some metal miter bars, I have ground some bars down that were too tight (with a 7" angle grinder.) Not as easy to make them looser.
There's other ways I make them also. I tried thin strip jigs, but thin strip jigs are usually for shorter stock. On longer stock... a challenge. On my panel saw, not a challenge at all though (very easy).
Hoping that info helps. If you have any other questions, just ask.