I haven't put a loft on mine yet but I have put a lot of thought in how I'd do it because I really would like one too. My plan was to use hardware cloth for strength and then put choroplast/plastic cutting board/plastic sheet from hardware store for the bottom of the floor and sides. I was going to make a completely enclosed box with the hardwear cloth and attach to the top and sides of the cage with zip ties every inch or so for the floor so there wasn't a risk of it falling. I was going to make one of the sides be able to hinge down so if I needed to get the hedgie out from the loft. I was going to use a pvc pipe or dryer pipe to access the top floor and cut a circle the size of the pipe into the hardware cloth to get into the loft. My cage is a little different but judging by the pics of the one you have it seems like it might work for yours too 