Marston's Rifle Holster

The rifle holster that I made for my John Marston (from Red Dead Redemption) costume, made out of foam core, Velcro, and (accidentally) two different types of leather brown paint. I carved a design into the foam doing my best to make it look like the in game version. Since the in game model is surprisingly difficult to get a clear picture of, and craft foam doesn't work anything like real leather, I had to improvise parts of the design. I've included the clearest picture of the in game one that I could take so that you can decide how good of a job I did.

The only problem I have with the thing now is that when I put my rifle into it the whole thing turns to lay horizontal on my back, not staying even remotely like it does in the game. Velcro doesn't seem to fix the problem, so now I appeal to the great costumers of DA to give me any ideas they may have to make the thing work as it's supposed to.