• general guide
• good choices on Amazon: [1]
• good smaller cages on Amazon: [1]

If I wanted to have an "upper" and "lower" cage (so they could escape the cat if they wanted, and so there would be a pipe for them to travel through), then these are the tubes I could use:

• 3" ID vinyl tubing $60 for 5 ft!!
• Masterkleer PVC Tubing, 3" ID $50 for 5 ft!!
• just bend PVC pipe