Actually 3W engines have the same requirement and what I've done with them is to make a lite plywood mount that is a bit too large in certain areas and cut it out in the middle for the carb. Then in the "too large" area drill for 1/4 bolts and use standoffs between the mount and the firewall.

I'd post a pic of mine but that cannot happen until tomorrow. I've seen tons of pics all over the place on doing this type of system

I did find this on AI's web site to give you an idea

http://www.aircraftinternational.com...ting/index.htm

Also, look at the assembly manual for the 75cc or the 100cc Aero-Works Yak. One of them (sorry I forget which one) shows a mounting plate they designed for the DA85