Actually no, but you're not far off. I'd like to beef up my firebox end legs a bit~ You got that right.
My manifold design called for it to extend only 14 inches into the smoking chamber, not halfway across, so there is not a whole lot of weight compared to a longer one.
It is heavy, and I can tell when moving the smoker around with the manifold in place.
The thicker steel does wonders with moderating the temps. It does take time to come up to temp in the winter, though. Once it is hot, whatever fuel cost was spent heating it up is well paid back during the smoke through heat radiation.
Here's a pic-
Never considered aluminum, since steel is so easily available to me. The more I think about it though, my instinct tells me to use steel.