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>