A couple of thoughts.... I'd probably not rely on the protective plastic as a masking medium for your painted 'frame'... suspect the solvents will do away with that quickly with rather unsightly results.
Plexi is notoriously difficult to paint anyway, i.e. getting the paint to stay put. You'll definitely need to sand/rough up the surface first, but even then....
I see the effect you're after, though, and it may well look OK if you can figure out how to do it in a way that will stick. Maybe vinyl striping tape stuck on afterwards?
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"