Steel enclosures

It's not used for exactly the reason you mention: something that will "ring for hours" is not a good material to use for an enclosure.  A good enclosure is acoustically neutral.  Metal is generally anything but that.  If metal is used for enclosures, it must be lined with MDF or dense sound-absorbant material such as Sonex, and then braces and reinforcements must be welded into it to prevent flexing.  In my opinion, it is not worth the extra effort, expense, or time, and the sound quality of the resulats are generally not satisfactory... and if it's the look of a metal enclosure you're after, I suggest building a standard MDF enclosure and attaching metal panels to the outside.

worked in a fabrication shop for 3 years.  In that time I installed a audio system in a fellas 2002 Dakota who also worked there.  This exact subject came up and he (being such a hotshot welder) said, "stealth box my ass, I'll just build one".  I tried to talk him out of it for the same reasons mentioned above.  He was convinced it would work.  We messed around with several materials and found aluminum to be the most acoustically dead.  Aluminum is a softer metal and does not resonate like other types of metal.  We tested this by taking tubes of approximately the same size and hitting them with hammer, tapping on them, basically playing with them.  Not very scientific.  Oh well, that was my observation, we never got the chance to build it as he moved out of state.  I wish we had done it though, would have been a fun project.

Yeah, the flexing is not really an issue with such short spans and thick plates.  I would use 1/8 to 1/4 inch.  Aluminum is also not magnetic, another bonus.  Will not rust, is soft enough to drill easily.  Its lighter.  The only down fall is price (and a little harder to weld, I'm a decent welder if your wondering). 

Well, if you build it, post pics.  Good luck : )