The sound probably has 2 causes: vibration of the bash plate itself and the engine noises that are now reflected upwards by the aluminium bash plate.

Using rubber washers would probably cure most of the vibration caused noise, but not the additional engine noise. I have known people that cured the engine noise with gluing foam for soundproofing marine engines on the inside of the bash plate. Any other sufficiently durable and heat proof foam should work as well.

If you want to get rid of all the additional noise I'd go for a combination of both the rubber washers and the foam.