The idea of the Swiss cheese look is to let light through, but I also need it to be rigid and not scare my wife and kids. So the center structure, to which we'll weld the treads, is a five-inch thick hollow shell with two ¼" steel plates for sides. We'll use the plasma cutter to create the round holes and put five-inch long pipe sections into them, overhanging each side by a quarter inch, giving us a lip to weld to the plates. We'll weld the mouse tower to a structural steel base and the landing to hold it all up and add a bit of reclaimed wood on top. That is, unless we change the design again tonight. And given that I don't have time to do a plywood model, the whole build is going to be one big dice roll.