Wow that's over complicated and bulky. I modified a Slik Sling last year and ended up ditching all but the straps. I added a snap-in swivel on spare strap, inserting that on the rifle forearm stud then adding the clips, as shown in the pics. Originally used the Slik Sling components to anchor it a bit more on my pack and to tension it if I wanted too.
Essentially, you don't need that stuff. Use a snap/clip set by anchoring one on your pack, the other end as I did with the swivel mount. If you leave enough slack in the strap, you can loosen and tighten as you wish. In my opinion, this method is much more secure, does not drift/slide off and you have no worry of the snap suddenly popping open as that light clasp may do. Pics show the pack with the Slik Sling strap minder still on it; you can keep it or remove it depending on how long the strap is or how much it annoys you. You implement this for a few bucks, buy or use any spare clips and straps you have.