washers work well enough they also will dig into the firewall over time you can use thin 1/16 aircraft ply, to make a spacer to fit behind your engine mount then you will have full contact between your engine mount and the firewall.
a little thinned epoxy and your fuel proof and good to go,I have done the washer thing on a couple of models now, mainly to remove thrust angle, and over time they all seem to dig into the firewall, when this happens fuel can get to the wood, and it starts to soften making the problem worse, really gradual so you won't notice it at first.