I've built many sleds over the years, several different types. You don't want a slippery surface, just the opposite. Often sandpaper is glued to the fence or base to prevent any workpiece movement. 3/4" birch or similar plywood is stable and thick enough to fasten fences, clamps or letting in T-tracks. I have a large rip sled must be 40 yrs. old and has been abused badly, made of 1/2" MDO, sign makers plywood with the orange paper, love that stuff. I don't make my miter or crosscut sleds adjustable, everything is fixed and dead on, with clamps. Rip sleds double as taper guides so they are adjustable, sometimes just by screwing down a fence or blocks as needed. Best thing I ever did with my sleds is using the Incra sliders, silky smooth and never change.