What size aluminum square tubing works best to fabricate a mount?

I guess I could also use two pieces of alumnum plate and stand offs too.

I have an aluminum gas cylinder that I thought would be great. Unfortunately ther isn't quite enough room for the screw heads that go through the plastic back plate. I may be able to pierce the side above the bottom of the cylinder on the milling machine to make room for the screws, I'll have to scratch my head on this. It will be fine for the muffler if I can't make a mount of it. .</p>

if you want to make a "waker engines" style mount, then measure from the backplate to the very end of the muffler, then add 1/2" to that.  on my ryobi, i just used stand offs. 

don't go out any more than you have to. The engine is long enough as it is.

All things considired the aluminum plate and stand offs give the best possible access to the carb and carb tuning linkages ect ect. standoffs are also pretty easy to adjust any thrust angle to as you just shim the standoffs.

I just used 1/8" thick aluminum strap that is 1 1/2" wide. I just bent it up in my vice, and it works very well. You can sort of tell in this photo what I did.Ijust ran maybe a 1/4" out past the muffler so as not to make the engine any longer than I had to. A real simple mount to make, but you still need to use a shim on the carb to get full range of throttle motion and also allow room to get the mountingstrap back behind that carb on the upper right mounting hole.

Cut a carb sized hole in the firewall and use very short standoffs.