For what its worth: I had a similar situation with a case I was relining for a Grant hgun. Here I had a little free space under the gun's wrist and, not wanting to 'bolt' through the leather wall and not having a thick wooden carcass to screw into, I solved the problem by gluing a baize covered wooden block to the case base and you can then stitch/screw/glue the hand strap to that. If there is no space UNDER the wrist, you could glue a block to either side of it and likewise attach the strap to those.
Hope this helps.
