The bottom should be doubled up with at least an inner layer of 8 to 10 ounce leather. Foam like trhese guys said, or if you are a true traditional diehard, a peice of unshorn sheepkin glued down there. The thick wool keeps the heads from banging around, and keeps them off the bottom. I've been using the same backquiver I made from bison hide for about 15 years with the sheephide in it. No problems yet.
Buddy, its best to have a leather spacer in the bottom so that the stitching never comes in contact with the heads. So you are lacing or sewing between three layers of leather, not just the tube and bottom. The heads then hit the leather, with the stitching safe inside it.
Backquivers arent for everyone, but I like them over any other.
This is the only pic I could find of it, in the upper left corner. Very simple, slim, light. Been all over hell and back with it.