If you make a good wooden or plastic model of what you're trying to sheath with the soft leather and maybe double up the layers with a good leather glue, you could try wet forming the item using lots of clamps or plenty of weight and thick padding like folded towels to press it. Let it dry completely, then wax it heavily. It should stiffen up a bit.
You could also try wet forming it flat with lots of weight before forming it. It will probably have surface blemishes and discolorations from the previous shape, but dyeing and waxing should cover most of that up.
Use the snot out of it and everything should blend together into a well loved piece of gear!