Here's some background on my question, before I actually ask it (my question is in bold below):
I'm wanting to have a very sustainable method of turning grains, legumes and pseudocereals into flours (which methods do not require electricity or reliance on corporations in the long run). In the short-term, I don't mind purchasing some materials. A variety of methods have come to mind:
Grinding with rocks. This is essentially like using a mortar and pestle. It works (I tried it with some club-like rocks I found in Kentucky), but most mortars and pestles that you can actually buy are very small, and unlike the sorts they might use in the East for grinding larger amounts of flour (wherein the pestle is like a meter long or such)—I haven't seen these available in the USA (that's where I am). Using rocks doesn't seem to work very well for larger, harder things like chickpeas, but it's very effective for millet, and works okay for rice and wheat. It's easy to squash the wheat, but requires further grinding if you want more than squashed wheat.
Using a hammer or mallet. I tried this on chickpeas and it works fine. I assume it works for other legumes, grains, and pseudocereals, too (although it may or may not be the most practical solution for tiny seeds like millet). Lacking a mallet, I could always use rocks to smash (rather than squish and grind them as with a mortar and pestle) them. I have a blacksmith hammer on order to try for this purpose, since its broadness should smash more seeds at a time.
Using a manual grain grinder. This is all well and good, but they may be a lot more difficult to make yourself (and might lead one to be reliant on others to get the grinder). Plus, really good ones are very expensive.
Making a huge animal-powered stone mill like they used to use. This would probably be rather effective and ideal, but I'm looking for a simple solution that doesn't require animals or things so large (and one I can tote around with me if I travel).
Anyway, my question pertains to item number 2 (using a hammer/mallet/rock to smash them). I basically want to get a very broad mallet, and a sack that won't break if I hammer it (I want to put the grains/legumes/pseudocereals inside the sack and hammer them until they're flour; the sack is to keep them in place while I hammer them). So, my question is, what kind of materials would be suitable for this sack? I don't want it to break apart when I hammer it (like the experimental paper I used with the chickpeas did).
I thought about leather. That sounds like it could work, but would it be treated with stuff that would make the flour toxic? Would leather be strong enough? Would something else be more ideal?
I'm also thinking about just using some old jeans or something to make a grain-smashing pouch. Maybe that would work.
Edit: I recently had the idea that if I'm just looking for something that I can do right now, with purchased materials, I can always just buy a packet of socks, and when one wears out, use another one, heh, heh. Socks are pretty inexpensive, but they would make me reliant on sock companies, probably.