I have a similar installation to JohnB-oak bearers approx 4 1/2 × 3 onto steel bearers (Engine is Sabb 2J).
As it was my own setup I decided to try sandwiching some 100mm x12.5mm Tico between the two- which is a vibration absorbing mat for heavy machinery. This was actually suggested to me many years ago by Tony Tucker, a well known marine architect/surveyor and a man who I hold in high regard as knowing his stuff.
I cant say for sure if its actually better or not as havent tried it without- but my installation is certainly alot smoother and quieter than many modern installations with rubber mounts and the washing machine produces far more vibration through the boat than the engine ever does!
https://shop.deltarubber.co.uk/tico-s-anti-vibration/tico-s-antivibration-mounting.html