The sensors are actually patented (in various countries by various entities including Konami and Andamiro - sometimes both). You should be able to pull the patent data if you can find it. I remember it has very detailed diagrams. DDR and PIU sensors are identical aside from the length of the sensor. Heck, DDR and PIU pads are almost identically constructed. I've heard from a few seemingly reliable sources that they started out as the same game. I almost think it's an off-the-shelf part from somewhere, but I sure can't find them sold for any other purpose.
Basically, it's that carbon footprint stuff like they use in remote controls. There are two strips of it running down the sensor on the top and on the bottom with a little rubber spacer to create an air gap. When that spacer is compressed, the strips touch, and the sensor is activated. Some friends and I chopped one up a long time ago, but I don't know if we kept it.
The panels themselves are just a 1cm thick chunk of Lexan or similar acrylic. Artwork is screened onto the bottom.
There are some plastic stops in the corners. It appears to be custom machined for this application. I'm not sure what the material is. It seems like a soft acrylic.
The rest is just a bunch of steel stock that's been machined in various ways then welded together or fastened with screws. The only real piece of interest is the so-called "L-bracket" that sits over the sensors to trip them. It rides along three spacers that are held in with screws and go through slots. This allows it to travel up/down freely while holding it otherwise in place. PIU additionally puts a plastic triangular retainer over these screws to help keep them from falling out (and it appears to work). On top of this "L-bracket" is a piece of foam attached with adhesive. Apparently, replacing this foam can go a long way to restoring function to worn-out stages.
I just recently tore mine down and cleaned it (when I finally got it set up a few months ago), so I don't really feel like tearing it back apart right now. I need to get some replacement parts put on order, and maybe I can grab some pictures when those get installed. May be a few months. If you ask Ho (here or IndyDDR.com) he may have some pictures from his.