Don't really understand your problem (worked late and have'nt had my mug of coffee yet),I use a transom saver that I picked up from Cabela's for a 20hp short shaft and have no problem with it bouncin' around. Turn motor to either port or starboard side, put yolk of saver between fins of lower unit and secure with bungy cord that comes with unit. They can be purchased in different lengths and some adjustable to meet your needs ie., long-shaft, short-shaft, etc..
A transom saver in my book is a must, that engine will put a lot of stress on your transom (sure your aware of that)especially with it bouncin'. Don't know if this novel helped ya', but hope you resolve the problem. Have a good one. Outta here... ~Skip