if my back of the envelope arithmetic is correct, the nominal diagonal of 4x10 and 5x12 are as follows:

4x10: 10.8 inches ( 274 mm )

5x12: 13.0 inches ( 330 mm )

So, none of the 210mm plasmats will cover the 5x12 format well. Most of them have image circles around 300mm - well shy of the 330mm nominal diagonal of 5x12.