The JB epoxy works well when used properly.
I have made four so far and learned through trial and error that you can't use too much JB. Similar to: you can't put too much water in the Nuclear Reactor. How much is enough? If it doesn't vibrate apart it is enough!

Seriously though, do not use it sparingly as excess can be ground or sanded off. The two important things to keep in mind are: Extreme vibration and heat cycling. A 1/4" fillet around mating pieces is about the minumum I would use for pieces that do not fit together tightly. If a tight fit is already had, a smaller fillet (1/16-1/8") can be used. The size of the fillet is also determined by the strength of the surrounding materials. If one or both (maybe more?) of the surrounding materials are thin use a greater amount in the fillet as vibration will be a major factor. Another key to a good bond is roughing up the surfaces where the JB will attempt to adhere. If the surface is too smooth or hase a coating on it the JB will not hold.

The inverted muff below on my TT Pro 61 was made out of some aluminum tubing that was squished with a vise and I think AL wind chime tubes!. The end caps are about 1/8" thick with a groove ground near the edge that the can fits into. A through-bolt holds them on. I made a one piece AL bracket that is inside which holds it on using either 8-32 or 6-32 through bolts. The bracket is basically a 1/4" thick muffler extension that has threaded holes. This was done so that pressure all the way around the opening would be exerted on the muff to exhaust port surfaces. The exhaust tubes are partially plugged with 3-4 layers of SS door screen molded around the tip of a finger and pinned in place. The screen offers a little back pressure and takes the crackle out of the exhaust note.

The other standard muff is on an old Fox 45 ringed engine and was made out of a piece of 3/4 AL electrical conduit. Grooves were ground in the ends and AL disks were peened in place. The exhaust tubes are only about 1/4" ID and were fitted VERY tightly. The assy is held on useing two muff extensions I made and through bolts as seen below. A flat was filed on the side of the can to properly mate with the extensions. Nothing plugging the tubes on this one.

In the past I've used the AL cartriges that power nail guns - good stout stuff and just the right size for a 46 size engine. I would imagine that mouse cans would work just great too as they are used to make tuned pipes. I wonder if those AL beer bottles would work...?! Might be just the right size for a gasser!
Sorry, all my experience has been with methanol breathers but the ideas above should work for gassers too.