Again... Excellent post. Your explanation about the different materials makes perfect sense. I know the primitive basics of the process but that's about it. So my thoughts are that I have plenty of the extruded aluminum, scrap steel and plywood. I know the steel would probably do most of the "grunt work" but unfortunately most of the pieces I have are pretty rough. It's used to crate up equipment and the manufacturers aren't really concerned about straight edges or cosmetic. It's strictly structural for shipping. There's a lot of warpage from welding and cranking down bolts. It's sturdy and strong but not precise. That's why I feel the need to have the mixed materials. The steel alone would probably take to much work to use alone.
When I refer to the plywood as bulk or filler part of what I mean is that I could see it being handy to use for anchoring and joining. Instead of trying to attach steel, aluminum and rails to each other I could hopefully get by screwing everything into the plywood and using it as a common base to build on. The aluminum would be mostly for the flat surfaces, steel for strength and wood to help hold it all together. Either way I suspect the mix would help with vibration. The steel angle iron doest take much to make it "ring". The plywood and even aluminum are more of a thump.