I think I over engineered my fix. I might take a piece of leather shoe lace or black nylon cord. Tie it into a loop about an inch and a half in diameter. pass the loop through the bar on the back of the dirk and then the frog through both ends of the loop. I would adjust the size of the loop so I could just get the frog strap through both ends. This would pin the bar of the dirk against the frog strap and cut down on the string.

A quick simple fix and no tools needed other than a sgain dubh or other knife.