Well it all depends on what you are using to cut it first off. I have used a plywood table saw blade and a jigsaw. Both cut pretty well. Then I have sanded using 600, 800, then 1200 grit sandpaper using a flat block to hold the sandpaper so you get a good flat edge. However, I have read that you don't want to sand any edge that will be solvent welded too smooth as the adhesive won't have anything to "grab" to. If you are using 1/4" thick plexi, I really doubt you want to use screws. You should find a better way to bond the two pieces if your current method isn't working. Tonight I will be doing the final pieces of installation of the case. I will also be working on extending the psu wires. That should be loads of fun. I'll try and post more images tonight.