Actually, plexi can be really difficult to work with if you're unfamiliar with it. It has a tendency to shatter when cut at too low a speed and to melt when cut at too high a speed. You're better off scoring and snapping it. Home depot sells scorers right next to the plexi.
You have to use a glue specifically for acrylic. Weld-on is the standard. Most people use a #4 and a #16. #4 is very thin, and works well for the intial gluing. Afterward, use the #16 which is much thicker to fill in any holes. You may have trouble finding it locally, but you can order it online at
www.craftics.com.
Check out
www.garf.org. In the DIY section they have a calculator and instructions for gluing your own sump.