I would build it out of steel and powdercoat it.
1. The application is for the lighting system and heat rises, the odds of it being moist are slim and none. So needing the stainless or aluminum for corrosion resistance is not needed.
2. Some low grade stainless does rust and aluminum oxidizes.
Unless you plan on polishing them....you will need to either clearcoat or paint them. I find most clear coats will eventually flake off or pit. So, getting that nice polished look from fresh aluminum or stainless to stay would be hard. There are some powdercoats that do look like fresh aluminum if you want that look.
3. Most people have a MIG or the access to a MIG welder.....welding mild steel is easy to accomplish and anyone worth their welding could stick it together. aluminum and stainless are a little tougher to do. Aluminum requires a MIG with a spool gun or a TIG welder.....and that is a skill in its own most don't have.
4. Mild steel is cheap and plentiful compared to the others and available readily in all shapes at places even like Home Depot or Ace. With steel you can choose a tube(round, square), angle, plate as well as materials like expanded metal in you design that would be harder to procure with the other materials.
5. Powdercoating comes in a multitude of colors and textures and is VERY durable.
6. With Mild steel you can possibly minimize your design for slim and a low profile look, where possibly if your pushing the material properties for tearout and deflection that same geometry would have to be beefed up and create a "bulkier" structure with aluminum. The same design in stainless, would just weigh more cuz stainless is heavier per pound than steel. An aluminum set up may look cool but could be easily bent.
For cost, ease of material procurment, machinability, finishing options, and maximizing design effieciency.......but mainly cost of fabrication and material......I say steel with a powdercoated finish for a straight foward and durable product that would last longer than the tank will.
