For those interested herewith a pic of my template for the MVVS 45 MG this is the size and shape for fixing within 20mm of the bulkhead any further forward the hole could be smaller as it will not have to pass the throttle stop or mixture screws (two slots on left) or the throttle operating arm (single slot on upper right).

Sorry forgot to mention this is for an inverted installation, I know this should be obvious but you never know if someone may cut before they have the engine.

Mike