I think for some reason the grille piece is pushing the grill up. It test fit fine before I put the grille in. Kinda bugs me but I'm moving on. The weird thing is this is probably the 4th one I've done and none of the others did this, so clearly I messed something up somewhere.
I fold the sandpaper and run a bead of super glue in the V, then flatten it firmly until it dries. It keeps the edge sharp and makes it a little stiffer. Just take your time and sand slowly and carefully.
That is a great tip.... I ave to try that. My corner always folds over.