I learned this trick from ChuckAuger and it works great.

Get yourself some G10 fiberglass material from an electronic/industrial surplus place.
Cut to shape and laminate several layers together using CA to get an appropriate thickness.
Using a sanding block, taper the flat edges to give yourself the offset you need.
Drill and mount. Hard as a rock and structurally sound.

Just takes a few minutes to make. You will never use washers as as shims again.
I have retrofitted all of my profile planes this way. This would work for firewall mounts too.
Here is a picture to help you visualize. The shims are under the motor beams.