Cant recall the names but Ive bought PGs from custom PG shops online. You could order them undrilled, or if you send them a template they will drill them for you, to fit your modifications.
Custom PGs also provide an alternative to painting on the top surface. A clear PG allows you to paint on paper or some other substrate and put your painting under the PG. You cant do watercolors directly on plastic, and you cant feed a PG thru an inkjet printer.
Find a photo online of a real live tortoise shell and print it on paper, trim it to the PG shape, frame it under a clear PG and youve got a unique version of the classic "tortoise shell" PG.
Another approach is to paint the under side of a clear PG. If you do that, I suggest a barrier sheet between your paint and the paint on the body. Bakers parchment paper would seem a good choice for that.
Heres some stuff I found via google:
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