Hi Will, not sure how much too big your rego plate is, but I cut about 15mm off each end of my plate to make it fit inside there perfect.
I used a dremel to make the cuts, and if you didn't know it was cut, you wouldn't see it.
I didn't cut into any of the numbers, so still legal.
PK
Winner, winner, chicken dinner!!! This is probably the very best solution of all. Thanx, PK!!!