We have 3 test machines here and all 3 have a different length J panel. So on the ones we sell we used the "middle" length. We have sold 1000s of these and for the most part its been a pretty good fit. With that said, some have been anywhere from 1/16 to 1/8 of a inch too long. You can resolve the issue in a few ways
1. You can loosen the side screws on the cabinet and knock it apart and drop in the J panel then retighten. For the center area only retighten until it hits the J panel
2. Simply trim the J panel. This stuff is easy to cut. Lay the old one on top and take a razor knife and score it on the BLACK side about 4-5 times. Make sure to get the edges good. Once its scored snap it off.
3. If you got power tools use a jigsaw. Go full send 
4. Request a RMA and send it back for a refund.