Oh yeah, a possibles bag.  I like the idea.  You can waterproof leather fairly well these days.  
A lot of people these days think a possibles bag was what muzzleloaders kept their reloading supplies in.  Not so.  A possibles bag was the original psk/bob.  Old timers kept a lot of important stuff in there, like firestarting tools, food(pemmican), letters from home, and yes, also sometimes a separately packed reloading kit and tools.
Leather is more resistant to fire than nylon, and holds up to UV better too.  I like leather. 
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The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)