The motor in the 500 should be glued down but I didn't fancy just sitting it on the plastic mounting. Damping out as many vibrations as possible. That's what they do in high performance motor cars so I suppose it's what I ought to do in my racing boat.
The damping is provided by a sheet of rubber bought from the local bike shop for a couple of quid. I could do three boats with the amount I bought so it's a good deal. Ask for giant puncture repair strip if you fancy obtaining the same stuff.
The rubber is self adhesive on one side and ribbed (stop sniggering at the back) on the other. That ought to stop the motor rotating around it's axis. Retaining the motor is now down to a few fat elastic bands which hook over those wire handles. I better follow a postman to see if I can replenish my supply.