Thanks everyone for all your help and suggestions.
I went with a "box" shape and was able to just use acrylic sheets and solvent-weld them together at right angles. I wound up making a box that is 18"x48"x4". It was made up of 4 18"x24" panels that I solvent-welded...
What I wound up doing is changing my design to eliminate the need for the metal channels. Instead of going with a triangular shape, I went with a "box" shape and was able to just use acrylic sheets and solvent-weld them together at right angles. http://apengr.com/AboutUs/TheVadenCompany.aspx
I believe that Industrial Galvanizers in USA are one of the biggest metal/steel galvanizers. They also specialize in hot dip galvanizing. You can check out their website below:
http://www.ig-usa.com