Hmm... not sure how well your concept would work. Because the vibration force is rotational, seems that the round shape absorbs the vibrations more evenly.

If you're in a hurry, my suggestion would be just to make a good, solid firewall and hard mount it. I think this is what most of the guys are doing with the EF Extra. The SAP-180 seems to have a very well balanced crankshaft and has very low vibration levels compared to other engines in the class.

The main reason I am using a soft mount is that my engine is in a pattern plane, where noise is more of a concern. Also using the tuned pipe for quietness and a bit more torque for a higher-pitched prop.