Originally Posted by vet57air
...would there be any reason to not make one from Aluminum?
I would suggest to you to use stainless instead of aluminum - aluminum is very soft and more prone to scratches, dents & dimples and it oxidizes very quickly, especially with soaps & such. Personally, I'd hate to go to all the effort to build this this thing and have it look cruddy in a couple of years.
We went with the stainless after removing the original aluminum side panels. The pan was stainless, but the walls were aluminum which was a good example of the difference between the two. On our new shower, the pan is welded - as the walls could be if there was a need, but we used the curved exterior Zolatoned walls and flat side walls. The only caulking is where the pan & tank cover come together (Vulkem between the sheets), top of the pan at the wall & at screw/rivet penetrations (you would most likely have these even with aluminum). I certainly wouldn't use normal bathroom caulking - the Vulkem is heavy-duty stuff, I don't expect to ever have to replace it in my lifetime.
Based on the longevity & other benefits of stainless, I think it outweighs any of the reasons you may think there are with aluminum. They only
benefit I can even think of with aluminum is maybe cost - but our whole shower in stainless - even having the pan professionally welded was about $450 as I recall. I can't imagine aluminum being much of a savings...