Almost all the plastics currently used as windows are safe for use in aquariums, they're designed to act like glass itself against water, light, heat, pressure and chemicals.
Regular plexiglass is just normal acrylic sheet... if you happen to be masochistic you can use the harder stuff: polycarbonate... but lexan has bisphenol-A in it and can affect your fish.
When I make flush-tops for tanks using plexi I use 3mm polycarbonate. For all practical purposes it is as hard as iron and just as difficult to work.
You can use hack-saws and drills to tool both materials, but be sure to get blades with a wide camber-wave on them so you don't bind up a lot. (the line the teeth are in wiggles widely from side to side instead of straight like a knife edge). The regular acrylic will do everything you need as a divider.
My flush fit polycarb lids will hold up a ten pound cat!
Keep in mind that if your hand slips along the cut edge it WILL rip you open. The material is safe-ER than glass but you can still make holes in your hands. Polycarbonate can hold an edge sharp enough to cut through leather and cannot be broken from just being scored unless you have a pane-breaker rig, trying to do it by hand can result in serious injuries and/or youtube video footage.