I have an angora rabbitry and wanted to allow them access to the grass beside the cages and the ability to stretch their legs. I used to pick them up out of their cages and physically bring them to the enclosure, quickly shutting the gate behind me. I was sure there was a safer method so made them a habitrail (or rather a rabbitrail) instead.
Step 1: Make a door
it's difficult to see from the picture, but I had to make a door on the end of each of the cages. I cut a piece of wire grid big enough for them to exit to serve as the door and cut the hole in the cage slightly smaller than the door. Secured the door with wire at the bottom to serve as a hinge. The doors are secured at the top with dog clips, and their water dish has two hooks on both corners to further secure the door in the event that one of the clips isn't properly secured.
Step 2: Find a large tube and cut a section out of the top
I polled folks on craigslist to find a suitable tunnel. The one I found is 15.5" in diameter and was about 20' long. The person I spoke to agreed to cut it into manageable lengths so I could put them in my truck. The PVC tube was very thick, so I cut a great deal out of the top to reduce the weight, allow ventilation and permit the rabbits to see above it. The tube was given three legs and was set at an angle to prevent the tube in step 4 from being too steep. I lined the tunnel with a strip of carpet to prevent the rabbits from slipping. (The wood structure to the right is an add-on cage I made when the new litter was big enough to need the space.)