How to make a cut-out on an acrylic sheet?
As you have already found out, cutting P-glass is tricky. The plastic cutters you mentioned are very gimmicky and don't work that well. I would suggest the following procedure:

After marking out your cut on the plexi,  take a scrap piece of any firm material, plywood, an old board, anything, and cut a slightly oversuninstallized hole in it to act as a backer for your plexi.

With your plexi on top of the backer, Drill holes in two oposite corners of the marked hole you want to make. 

Use a varible speed jig saw at a very low speed with a fine tooth wood  or course tooth metal blade to cut out the shape. Stay just barely inside of the waste side of the mark. 

Finish the hole and smooth the edges with a fine bastard file and some 220 grit paper.

If you are really cleaver, you could make the hole smaller than the device cover so no plexi edges would be exposed. You may need longer screws to attach the cover plates, but you can get them at any electrical dept.