This is what I have done.

I use a 1/4 thick piece of plywood as a firewall
and then mount that firewall on standoffs. It's the
same way most gas engines are mounted.

I make my own standoffs but someone offers them
commerically but I'm not sure who.

I usually use aluminum for the standoffs but if I need
some weight in the nose I 'll make one out of steel.

Regards,
Roby