Sounds like you are trying to pour in dry primer, then dry glass.
Consider instead: make silly string out of the primer/glass/binder, and while still "wet", use a razor to cut little #10-type circlettes, place those in the cup, and they will tend to stay in place. Top off with red gum or shellac or other fast-drying water-resistant sealant.
This is why two pre-made cap-gun caps are the preferred source of ignition. Much simpler. Not necessarily better, but definitely simpler.