Dickeybird has the correct answer. Don't CA the parts to the glass---hold them down with lead weights. That is how Dave Platt does it in his video tapes. I made a mold from GE RTV silicone using a plug made from balsa of the shape I wanted. Then cast them from melted lead ingots. You can find those at plumbing supply houses or sometimes at your local hardware. Or, some have suggested visiting a local tire dealer and asking for their used tire balancing weights. Regards.