Hi Kevin
The Robot Rules (section 8) explains what materials you can and cannot use. It's kind of complicated, so please take your time and read it a couple times and see if you can figure out your situation.
http://usfirst.org/community/frc/content.aspx?id=452
Also it's usually better to find some polycarbonate (such as Lexan) to use instead of acrylic (Plexiglass).