If you want to use 1/2" ply, you can get a "T" slot router bit and rout some "T" slots in the 1/2" ply.
Even in 1/2" ply you will have enough room to install "T" track in a 1/2" piece of ply in a sled application. Installing the track would weaken the base of the sled, but with the fence and rear support you would get your strength back. Remember a sled has a kerf sized cut all the way through the base, so "T" tracks aren't going to hurt it at all. The only difference you be, you wouldn't have much material left under the track for a screw to grab onto. You could just glue the track in place, or install recessed t-nut under the base to accept some short screws. You only need a couple of threads to make contact since there are so many screws used to hold the track in.
Mike Darr