Take some 2mm leather, matching thread plus a few tools and you too may enjoy the freedom that is a non shoulder slipping in-the-field rifle:
The most challenging element is getting the correct fixing position & orientation*prior to stitching in place as such will only be realised with your chosen jacket on and rifle at shoulder as you intend to carry. Therefore you'll need help from someone with patience and the ability to temporarily secure in place prior to gluing. (*Note: Its important that when rifle sits at the point of the shoulder you want it to you ensure the stacked leather tab is square to sling edge in accordance with how it wants to orientate itself.)
Sadly the stitching is an absolute pain even if you're familiar with saddle stitch and this install took me some 3hrs - the need to keep coat lining from shifting prior to each awl mark accounting for half of this!
You could of course attach c/o 4 or 6 point riveting but K doesn’t do simples.