Thanks, jdurand!
Q: -Being into pyrotechnics yourself, you surely must've used bentonite clay for tube ends and nozzles?
If you press it in firmly, it's almost as cast - then seal off/ fixate with a droplet of waterglass.

Cheapest glue there is, also good for fire protection of just about anything fibrous (but it gets stiff and glossy, tho).
I use it by the gallons all the time at work - fireproofed a complete stage set entirely made from cardboard (sic!).
Painted/ waterproofed a concrete base for a textile dyeing machine by adding 2-3 teaspoons of atomized aluminum per litre,
completely filling all the pores, looking great and easy to clean. Ain't no dye stains no more!

Cheers
Johan