Plexiglas is acrylic
Plexiglas is a brand name for acrylic plastic. Specifically acrylic sheet is polymethyl methacrylate (PMMA), Plexiglas is probably the best known brand of acrylic sheet in the US.The question is why are you replacing the lights (windows)?If they are leaking, they might just need rebedding. If they are scratched, there are several acrylic scratch/haze removal products on the market that are excellent. If they are cracked, damaged, or you want to change to clear or tinted lights, then you should consider that the entire light needs to be replaced.The lights are integrally constructed not only of the clear/tinted portion but also the exterior and interior coamings. D&R Marine (508-644-3001) (holder of the O'Day brand) has a local fabricator that constructs replacement O'Day lights. They are somewhat expensive, but provide for a watertight (if you properly bed them) OEM replacement.Use silicone (marine), not polysulphide when you rebed them.