I’m eventually going to get a Tunnel Haven, because I’d like to be able to make turns. It’s a very clever design.
In the meantime, I did what JerseyGirl and Elrohwen suggested. I bought a cardboard concrete form at the hardware store. It was 8′ long and cost $10. I had them cut it into 3 pieces, which I rearrange daily for Borys. He loves playing “Tunnel Bunny,” and will run through them in excitement when I enter his pen.
I also created a tunnel/warren in his cage. I turned it into a two BR by putting two boxes side by side, and cutting a hole in each so they connect. One has a hay nest where sleeps, and also eats. It contains the hay until he chews too much out of the side. Cardboard, thankfully, is free and replaceable.
He doesn’t seem to tire of hopping back and forth. His “spare bedroom” has another hole that backs onto the second door of his cage. So when I open both doors to his cage onto his exercise pen, he has a lot of fun running various patterns through the holes and over the tops of the two boxes. I made sure to use sturdy boxes with solid bottoms. The kind stores use to transport glassware and other heavy, fragile items. I flip them upside down. The roofs hold up even once Borys has chewed away two whole sides, and he also likes to sit on top.
His cage is a large size dog crate, which comes with doors on the sides and one end. Bunnies love circular patterns, and get pretty creative running in various directions.