I don't think Ken's will help you, as my memory is that it uses a rear mount toward the driver's side of the engine.
The MGF is the same:
http://www.mgfcar.de/epc/2653.htm
Your best bet must be to use longer bolts between gearbox and adapter plate, and connect that way. You'll still need a fairly sturdy bar to brace the engine to though, as it's quite a width to span - I've got roll-cage spec CDS spanning the rear of my chassis, and it certainly flexes under load.