I'm working on a similar project and the owner of the gun is concerned with retention, so for now we've agreed on a thumb-break.
I see there is no retention feature built into your design (that I can see, anyway).....is that because the gun is big enough that gravity will suffice?
Also, we discussed the gun sliding through, so we decided to simply stitch the bottom closed. Was there ever any thought given to that? The screw seems like one of those "just another part to worry about" things.
Thanks,
RG