Plexiglass??

[Syst3m]

Goto home Depot and ask for an Acrylic knife. You Buy a Clamp if you need one to hold your square in place and score the hell out of the acrylic with the knife, It needs to be about half way through the acrylic, then you snap it off on a level flat surface using the corner of the surface/counter as your leverage point.
If you can practice on some scrap peices thats best. the only reason I dont suggest dremel for the job is the amount of plastic thats going to be tossed into the air and the time its going to take
not to mention unless your steady as a rock and can move your arms like a machine your going to have a hell of a time cutting out anything large. I usually try to get my glazier to make my pelxi to size. Thats the easiest way.

Good luck

[Kurzix]

I have no problems getting good square cuts on plexi, granted it can be a pain...and you don;t eve3n need to score it 1/2 way...heck the direcetions only say about a 1/3...but the more you score the easier the break I imagine if you went to 1/4" or more thick you might need a mallot to break it

[naja]

sophia... try a jigsaw on the plexi. a skillsaw will usually get all gunked up with melted plastic (plexi), and though the jigsaw will also melt the plastic, it is easier to stop and clean out your cutting point. just make sure that the sheet of plexi is being held firmly on both sides of the cut and you have a sharp blade in your jigsaw, or, use a table jigsaw or a bandsaw. ciao!

[Seanebones]

I wouldn't use it.

[lance]

well howed you make out did you go plexi or glass and how did you cut it
tell us we want to know so we dont make any extra mistakes