For a piece that's not flat (ie, a dash bezel):what's the best way to make an accurate, clean square opening in a sheet of ABS?
Tape off the edges so you have straight lines. Then score the plastic along the edge of the tape with an X-acto knife. Next take a dremel tool with a cutoff wheel and cut along your lines (the scoring will help keep your cuts straight.) Next get some flat files and use them to straighten, shape and smooth your cuts. They work much better than a sanding block or anything like that.
After some patience and TLC with the files, you can have very nice straight cuts.