For larger panels I like the "Norm" style sled that only has one runner and a fence on the front. I don't think these are as accurate as the dual runner style that is referenced off of the back fence instead but for larger panels I don't think this is a big deal.
The way I made my last one is as follows:
1) mill a 11/32 x 3/4 strip of hardwood for the runner (just a little bit less than the depth of the miter slot)
2) put double stick tape on the runner
3) place the runner in the miter slot with something underneath it to raise it a little proud of the table top (1/16?)
4) place a piece of ply (20x36?) over the runner. The ply must overlap the blade by a hair. It will be cut flush by the blade in the end. And try to make the ply square to the blade.
5) take the ply off carefully and attach the runner with glue. I have done this a couple ways and have never been 100% happy
. Last time I drilled pilot holes and screwed it down. Then unscrewed it, applied glue and screwed it back. I also set it back in the miter slot to dry.
6) after it has dried run the sled over the blade. This is your reference edge.
7) Drill some 1/4" holes approximately square to the edge of the sled. Flip it over and recess the holes and put in some hurricane nuts.
8 ) get a good reference edge on a piece of hardwood using a jointer. The hardwood should be the length of the sled. This will be used as the fence.
9) use the holes in the sled to mark the center point for holes in the fence. Drill 3/8" holes at the marks in the fence.
10) attach the fence to the sled via 1/4" bolts and washers. Square the fence to the edge of the sled and tighten it down.
Good luck!