Jeremy,
I like 7 or 8 oz veggie tanned leather for sheaths. 10 oz for larger blades. I personally prefer the thin 4 oz leather in an pre-colored oiled material. I particularly like a dark brown distressed look. You want a back quiver to collapse in the middle to hold the arrows in place and keep them quiet. Thicker leather tends to stay round or oval in shape so the arrows rattle around a lot.
How is your forge build coming along? Email me your address, I still need to send you that extra red kydex for scale liners and handle seperators.