I think if you have a very light, fan cooled sled, the plexi glass would work for a while, but I think in the cold it would just get brittle and crack, not to mention get gouged pretty fast from the carbides. A heavier liquid cooled sled would probably just drive the carbide edges right through the plexi. My experience with Plexi-glass it that it's fairly brittle and scratches easily, compared to the UHMW or High density plastics used in bed liners and the store bought deck protectors.
Maybe try and find some old worn out sliders (hyfax)? Screw them down with the mounting slot facing up, they would do a good job keeping the sled straight during loading and unloading. On my trailer, I'm just going to lay down some sacrificial boards that can be replaced easily, it won't help the sled slide any easier, but will save the deck from deep gouges.
Good Luck.