I went with wood, currently holding 4x 1080ti & 4x 1070. Mark off your first 8' 1x2: 36", 2x 7", 2x 11", 2x 11.75", approx no waste. Mark next 8' 1x2: 36", 2x 12.5", and you're left with ~35" piece for under risers. Wood cost $3, oak plywood $6, closet bar $4. You need 1 1/4" screws and I paid $1 for the wood rod. If I did it again I would actually use wood glue instead of staring at the bottle the whole time. Metal would be better, but this took me 2 hours max.
Wow, nice build, cheap and simply. I think with regular vacuuming it should not be necessary to wash the rig down (which would have been the main advantage going for a metal structure). Working with wood just seems easier. Will have to take a look.