>>64098128You can use all kinds of things as a counter charge. Again, you can use saltwater, or you can even use biscuits, see:
>>64098143It literally just have to be some kind of mass which the backblast can scatter, to absorb a lot of the energy, the only question is what would be a practical mass to use. Fine powders are ill advised due to all kinds of different ones being potentially flammable, but you don't have to use a fine powder.
If you're really dead set on it not being plastic, I would say you could do something like a long stack of some kind of round hardtack (clackclack) packed in wax or something, to make sure it lasts indefinitely (not so much for eating, just making sure it doesn't break down).
Also, unless you're looking at a reloadable piece like a Carl Gustaf (which has its merits and advantages, mind), you're not going to be moving away from synthetics for these kinds of launchers, the entire point of the typical launcher is that it's a really light and handy weapon to carry with you because it only has to tolerate being fired exactly once, and there's no better materials for this than plastics.