Blue 3M vinyl Fineline tape works well for the edge, then just overlap it with ordinary masking tape to attach a mask to the rest of the plexi. I used 4 mil plastic sheet inside and out, with a foam pad on the top so the whole assembly could be flipped on its head to work on the inside.
Black electrical tape is cheaper than Fineline, but may creep if stretched too tight and I'm not sure about solvent resistance when you get to painting. Whatever works. Either way, you don't need 2"; lay a strip of ordinary 3/4" to define the edge then lap over it with the mask. To bend around a small radius curve you might use an edge-defining tape as narrow as 1/8" and then lap it with masking tape.