This link is from another one of our users, I'm not quite sure who's plane, but thanks to whoever took the pic..It's a real nice illustration of exactly how to do it. I did mine just like this, and it seems like a very clean and lightweight set-up.
Here's the link:BME set-up
Inexpensive as well. It's the GreatPlanes 60-120 adjustable mount. I epoxied the mount together, then split it, cutting away the steering bearing and cleaning up the backside,as there is molded lettering that won't allow it to mount totally flush with firewall. If you use this you'll see what I mean.

Are you putting tank on CG? I'm also about 3/4 done with my Extra, w/BME50. This should work well, I see no reason why it wouldn't work.

I have a whole book of notes on this plane I've taken from all sites, if there's any tricks I missed, or any that you might want, PM me and we can talk. These are very nice planes that won't break the bank when it comes to GiantScale in my opinion.