I've got a cute, tiny carrying cage that I bought on eBay for $7, and it's big enough that you can squish 3-4 rats in without them complaining. I'd say two is an ideal number, though. http://cgi.ebay.com/MINI-TRANSFER-T...QQihZ008QQcategoryZ116396QQrdZ1QQcmdZViewItem
The thing that I've noticed about carrying cages is that the lower the ceiling, the happier they are to just lie down and sleep. Then they don't feel compelled to climb & hang upside down, trying to get out.
Also, I think any carrying cage needs to have a large door so that your rats can't cower in the corner, eluding being picked up (and wasting the vet's time).
There are some more rectangular carrying cages available at many pet stores for around $10-12, where the entire top opens. Those, imho, are a lot nicer than the one I got.
And having a nice plastic pan for the bottom is really important - you don't want poop & pee on your carseat.
If you make one yourself, I'd love to see it!