[morzh]

What I think could work is this:

You warm the plastic up (gently, of course).
Then warm up two glass panes. The bottom pane should be thick and rigid. Thinner ones can bend some.
Again, warm gently, should not warm so they melt plastic.

Then put the gently warmed plastic in between the two, and put something very heavy on top. Heavy to flatten the plastic but not to crack the glass. And something that is not conductive. Yeah, books.
This way the cooling will go gradually.

Let it cool to room temp. See if this is the kind of plastic that it works for.

Oh, and if you think that warmed paint can transfer from the plastic to glass when it is cooled and lifted off the glass, you could possibly use saran wrap, one unwrinkled layer, possibly at the bottom glass (then the plastic should go by the paint facing down).