you're not going to be able to score and snap plexi. the easiest way to cut it is to ask the people at the hardware store to cut it for you. you could also use a hacksaw, but it's a real ***** and takes forever. a jigsaw tends to crack the plexi real bad.

you can also cut it on a table saw if you've got a real sharp carbide-tipped blade, but you gotta be careful..

i've dealt with plexiglass more than my fair share, and it's a real ***** to cut.. i just have the hardware store do it now