I made an successful "Hyde" type mount from plywood and EVA foam. It ended up being over 7 inched in diameter before it supported the engine well. It was stiff compared to what is seen on pattern planes but did reduce some airframe noise. The problem that I see with softmounting big engines is in dealing with their high amplitude- low frequency vibration. Its difficult to isolate the engine very well and still provide enough support for it. If you have a huge firewall its easy to construct a good working and lightweight softmount. I do not think soft mount development will become mainstream in giant scale because for the extra weight and expense of making a single smooth you can get a twin cylinder and do just as well. Besides that, Nobody really cares.