Not that I know much about such things, but last month I picked up some new powderhorns ($ 5 each), and observed that the rear dome/plug parts weren't installed so as to be waterproof or airtight. My attempt at a solution involved taking some water-soluble-but-dries-watertight transparent caulk that I'd gotten at Home Depot, running a bead around the crack between the wood plug and the horn, also putting a spot over each of the tacks, in case there were any leakage through there, working the caulk into the cracks as well as I could, sucking as much air out of the powderhorn as I could, just to encourage the caulk to fill in better, and repeating the caulking once or twice, until I could get a really good vacuum pressure inside the horn. Soon after each application of caulk, I'd use a damp paper towel to dab off the excess caulk from the outside of the horn, taking care not to remove any that was filling the spaces between the horn material and the wood. After the caulk dried, I went over the whole horn with fine steel wool, partly to remove the "just bought at a modern store" shine, which also took off the several patches of excess caulk that I always missed with the paper towel. Results seem likely to be fairly water-resistant. One thing I didn't want to do was seal the horns with anything that might react adversely with gunpowder or powder substitute over time, hence my approach that involves very minimal (and maybe almost no) potential contact of the powder with the sealing material.
That said, I'm quite the novice at this, and I'd welcome anybody else's suggestions, cautions, cautionary anecdotes, etc.