Allow the assemblies to dry overnight, or at least for a couple of
hours. Drying time depends on how much thinner you got into the
seams. For the next step, the assembly doesn’t have to be rock
solid, but you don’t want things falling apart as you work with
them.

While waiting for the parts to dry, you can begin to make the filler
cement. Cut some of the scrap plastic from the moldings into little
bits, and place them into an old baby food (or equivalent) jar. It’s
important that your jar has a lid that fits well. After you have an
eighth to a quarter inch deep of plastic bits covering the bottom of
your jar, pour just enough dope thinner into the jar to wet all of the
bits. Cover the jar and wait an hour or two.