I haven't seen a three sided marquee so I might be off base...  but if it's just a triangle, I'd think a score-and-crack using the original plexi to get the lines would do great, even for triangles?    (no?)
If not, you can put Lexan through a table or circular saw and it comes out nice and clean, just takes a few minutes.
Or you can remove the artwork...  just making suggestions.  
