Jay,,,
I made a hardwood ring out of 3/32 ply and lightened it up pretty good. I think there is a pic of it earlier in the thread... . I just epoxied it to the backside of the firewall when I finally installed it and snugged up the socket head bolts in the blind nuts.. I used a lockwasher in the hyde mount and I didnt over tighten, just good and snug. I didnt at all get the sense that the firewall was crushed even a little.... I used a T handle socket head wrench to tighten them and it has worked out well through approximately 30 flights. I wouldnt think it would hurt to install a few dowels the thinkness of the balsa around the perimeter of where the shim will ride on the firewall, but I really dont think its necessary if you dont overtighten the motor mount bolts.