Dunno how the crowd would feel about alternative materials, but I've played with them without tipping my hand here and learned a few lessons. Biggest lesson is the importance of a good seam seal. Without waterproof seams, it doesn't matter what you build with. Along those lines, sewing inside out with a welt and reversing the bag sure helps dry up the seams, but I use SnoSeal on them too.
I have however, found a leather than I like here in our semi-wet country (semi- because we only get 90 inches a year compared to your WET country). I use
Tandy's Stoned Oiled Leather which is treated with waxes and oils. Still gets wet, but I go one step further. After construction I dose them hard and regularly with
Fiebings Leather Balm with Atom Wax. It darkens the leather, but goes miles toward keeping leather dry (ish). Might give it a try.
Here's a "wet weather" bag I made using that leather, followed with a dose of the Fiebings. The leather started a medium brown, and the Fiebings made it dark brown.
BTW- Get yourself a copy of TC Albert's great book
Recreating the 18th Century Hunting Pouch. It's the bible for bag building with incredible step-by-step photos and instructions for bag building. He also includes some patterns taken from originals. Just be forewarned that the originals are lots smaller than the suitcases guys carry today.
One more thing of note: In wet country I HATE big old horns that hang outside the bag on their own straps. Not only are they a great way to ruin a lot of powder, they tangle in the brush. I make my horns only about 6" long so they drop inside my bags, yet they still hold around 1,400 grains of powder.
More pics: