Make scrap metal into chunks of steel. (optional - can go directly to step 2)
Make chunks of metal into lump of steel.
Make lumps of metal into hard plating.
Congrats, you’ve turned 288 items (volume 288) into 1 item (volume 12). It’s easily the best way to carry metal that I have found.
I don’t know of a really good way to do that with rags, but a long patchwork scarf is 12 rags and only volume 4, so that’s something… anybody know of a better one that doesn’t take FOREVER to craft/disasseble?
Haven’t bothered trying with leather, and I don’t think you can really do that with Kevlar (closest thing is to not cut up or dissemble kevlar items you are keeping for scrap - the dog harness, for instance, is only volume 9 and cuts up to 18 kevlar).