First attempt at making a cornet mouthpiece adapter for the GVS Elipse is turning out pretty rough, and I'm not sure how to improve it.

[forth@neuromatch.social]

First attempt at making a cornet mouthpiece adapter for the GVS Elipse is turning out pretty rough, and I'm not sure how to improve it. I molded this in place in the Elipse, and the silicone sealant just didn't settle into place well enough. Also, some of it didn't set even after 12+ hours. Anyone have tips on DIY silicone molding?

If I could readily acquire a spare valve hole plug (also pictured) I'd just drill a hole in it and form a seal around the mouthpiece with silicone, but I haven't found anywhere that sells just the plugs.