Modified T/C belt

My Broncco Renegade uses a 5/8" symmetrical belt which seems to be made of unobtanium, the only belt I could find would creep because it was 3/4" wide and too short. What I tried was using an asymmetrical belt for a Manco(?) and drum sanding the angle so it resembled the symmetrical belt that was required. After 30 minutes of stop and go driving the belt shows no sign of damage or wear. Has anyone else done this, do you know if it will hold up? The T/C operates just as it should now and the creeping has stopped.