I use a thermoplastic product all the time to hold odd shaped objects like earrings, bracelets and pendants.
I believe that the product was called JetSet, but I will have to check on Monday. I am pretty sure that I originally purchased it as a kit from Stuller, that came with several pounds of plastic pellets, a hotpot to heat water in, and a small screened basket to heat the plastic pellets in..
I have several holders, from an aluminum GRS T bracket, to aluminum cups with a flange on the bottom, (also from GRS I think), plus a couple wooden holders that I created for an unusual application.
I can then clamp the holders into my GRS MiniBall, a larger Engraver's ballvise, or even a ring clamp in the GRS BenchMate, freeing up both hands to work with.
I drop the holder into hot water for a couple of minutes, set the object I need to hold into the softened plastic, and then set it aside to let it cool and harden.
When I am done, I turn the hotpot full of water back on, and drop the holder into it until the plastic is softened enough again to remove the item. I have reused, reshaped the plastic, and made adjustments several hundred times with each clamp by now, and have never had to replace the thermoplastic because it has failed or deteriorated with repeated use.
I use this technique for all sorts of situations where it is not possible to hold an item otherwise. I set stones in earrings and pendants, etc., and do minor engraving on small items this way.