If the plexi is flat they won't slide off. You could glue a piece of outdoor carpet onto it (AstroTurf looking stuff).
I use plexi for the ramp to the basking platform as well. For the ramp, I just drill a bunch of holes in it so the turtle can grip the ramp while climbing ot basking from the ramp. Holes in the plexi were free

, work really well and makes it easy to clean.