These were fabricated from some drawers from an old crappy desk. I reinforced the inside with a 2x2 and then put a tiny piece of trim, maybe 3/4" square on the back and drilled through both. I needed the tiny trim on the bottom because the bottom of the drawers are recessed. Everything is held to the wall with two toggles each. There are actually 4 drawers but I don't have a good shot of the entire wall. These are the top boxes, there are one more on each side so the cats get climb up like stairs.
The bridge is a salvaged piece of wood, some really odd dimension. I cut the ends on angles to match and fastened it to more 2x2 which is also cut to the same angles. I buried tee nuts in those 2x2s and used machine screws to attach. That way the bridge can be taken off if needed. We wrapped the bridge in rope, using hot glue periodically and a hammer to make sure it was tight, then using staples every could inches as well. The cats have beat the hell out of that rope but it hasn't budged.
