I ran into the exact same problem with my dirk and frog. I trimmed the sides of the strap to fit through the bar on the sheath. It works fine.

If I were doing it again, I'd notch the frog strap at the point where it would pass through the bar, then unscrew the bar, fix it in place over the strap and screw it in, again. A little Lok-tite or super glue would keep the screws from straying. You would not be able to easily remove the sheath from the frog, but why would you need to?

As you can tell, I've thought about this. Good luck.