I love to crochet and I have been experimenting with making thinks with recycled t-shirt yarn since I go through so many. So far I have made a basket like bed and this tunnel which I think I might hang from the top of the cage like a sleeping tube. All I did was make a chain long enough that when it is joined into a circle it's wide enough for my ferret to crawl through. Then double crochet in each chain for as many rows as I had yarn for. I bet it would work for connecting levels as well if it's hung on the side of a cage.
So many great ideas on here! I love the tunnel! I have so many old T-shirts laying around I ought to make some T-shirt yarn and learn to crochet.
It might be risky to use to connect cage levels because a ferret could dig through the yarn and escape.
The t-shirt yard is pretty thick and strong so I think you see any noticeable damage before any structural failures happen. The walls are almost a half inch thick. Unless you have a serious chewer it should hold up. So far it's just being used as a toy/hideywhole so we will see how much damage it can take.