I built my cage outta plywood then sealed it to protect the snakes. An easy way to do the plexi doors would be to trap a piece of plexi between two pieces of wood that compose the frame.
Essentially, make two square frames from say 1/2" material, lay a piece of plexi on one and glue it in place. Run a bead of glue on it and lay the other piece of frame on top and screw/nail the wood together. Fill the gap between the two pieces of wood with silicone and your done.