I would say use 3/4 material... why ? a 4'x2' is a decent cage size but would stand the possibility of failing after useage due to weight of snakes and other items in the cage ( water bowls,substrates etc.)by failing i mean "sagging in the middle" if the cage is built with a "face frame" it will support the overall span of the cage keeping it from sagging if 1/2" materail is used but you would want to "dado" the face frame so that the 1/2" material is recessed into the frame of the cage .I would say use safety glass due to the lexan/plexi will become marred or scratched with time from scales and debris ( substrates) and i would also suggest if you use glass pay the extra $ for the proper track hardware this will last a lifetime and also not be split,cracked or warped due to heat or the weight of the glass such as plastic tracks do.
