I was doing research on Pygmy Hedgehogs (yes, I know they are illegal in the state of CA *tear*), and I thought their owners have come up with a clever idea for a wheel! They take a bucket, and some PVC pipe for a stand and make a wheel! I'm sure with a little modification (the right sized bucket!) a rat owner could make something just as nice! Here's the link to the first page I found it on; (you'll have to scroll down a bit).
http://www.angelfire.com/wa2/comemeetmyfamily/hhcare3A.html
Here's another page with a little more info;
http://www.geocities.com/hedgiewheels/
http://www.angelfire.com/wa2/comemeetmyfamily/hhcare3A.html
Here's another page with a little more info;
http://www.geocities.com/hedgiewheels/