Unfortunately as you indicated, we do not carry that specific size and it is not possible to cut a FlexPlate System down to size. Each size magnetic mounting base has a unique arrangement of magnets designed to keep the FlexPlate level and in place. In addition, the metal FlexPlate requires an industrial laser for a clean cut.
We do have a 406mmx406mm size, and as long as there isn’t anything obstructing a slight overhang, should work with your system.
Let us know if you have any further questions and happy printing!
Regards,
Dave