A soft mount will let allow any engine to move (vibrate) A hard mount fibreglass or aluminum should solve the issue assuming of course a balanced prop martin
i have had no issues using both on 2 and 4 strk or diesels martin\\

They do make an iso mount which are rubber bushings with bolts to secure the hard mount to the fire wall which works Davis Model products in the US sells then very easy to
install understand they work well