I use industrial strength Velcro often. It works for me with belt loops, as well as flaps on folder sheaths.
For the belt loops, the loop is two-piece leather, one from the top overlapping the lower one, with Velcro holding them together. The 'shear strength' of industrial strength Velcro is amazing. On a folder sheath flap, a 'dot' of Velcro on a molded flap is more than enough to secure it. I also use Velcro on modular sheaths that I make, based on the old Gerber Gardian sheaths; belt, boot, and shoulder 'bases' that attach to the sheath with a full Velcro backing. Wonderful stuff.
Stitchawl