With the Homelite 25 and 30cc engines, I use studs to mount the backplate. The studs go through the engine backplate and the backplate is secured using washers and nuts. Then I place flat washers against the nuts, and the studs go through the firewall and are secured with flat washers and nyloc nuts inside the plane on the back side of the firewall. This is not only lighter than using a special mounting backplate in addition to the engine backplate, but also is a much smaller footprint on the firewall. (Less space needed.)

AV8TOR