Thought this might be of some interest, with the recent focus on LBE.
I'm not sure how exactly we acquired this, but this is a case for a camera flash. It's got a thin leather shoulder strap, and is otherwise made out of pretty durable leather. I've attached a few pics, feel free to use them.
I acquired it for my own use, and when I got my Ishapore Enfield(.308), it became an impromptu ammo case. As you can see from the pictures, a 5 round stripper fits perfectly, and they can be stacked back and forth, about seven layers deep, though I don't have that many stripper clips. That's over a hundred rounds, though I'm again not sure if it'd actually support the weight.
This isn't perfect, but it gives me an option for carrying my ammo around until I get a proper set of web gear set up. The strap will probably dig too much without padding, and without a belt to hold it down like you've mentioned a few times, it'll flop around a lot, but hey, it was free. You've probably seen a number of things like this in thrift shops that could be drafted for various purposes.
Another thing to mention are old pants. Legs cut off at a desired length and sewn shut on one end and you have a bag. Sew on some nylon strapping and there you have a shoulder bag. If you have cargo pants, watch your cuts--you can get the section with the pouch on it off in one piece, and that gives you an extra pouch. Pants that have the zip off legs can be cut off just above the zipper, and with that closed, you have an easily seal-able bag. A plain jeans or BDU leg can give you a decent mag drop bag if you like though.