Run the leather upright through the jaws. That way the jaws are holding onto a shorter bit of the curve.
This pony's clamp-face is almost square, mine is 3 inches long but only has an inch tall surface. If you run the strap vertically, or at an angle, you can glue the straps together as both Dwight:
and TomE did...
And the pony wouldn't (shouldn't?) put undue pressure to push the strap flat again.
At least, I think that is what TomE was getting at...
I am stealing Dwight's method for my next gunbelt. I'm happy as a clam to find him still here after all this time! I last poked around here about 5-7 years ago; I'm not good with the passage of years.