I'd like to thank everyone for their wonderful ideas! We found a kiddie trailer that had been converted to a pet trailer on Craig's List, so we bought that and are now searching for a small cooler that will fit nicely inside. I have to tell you, when I told my husband what I wanted to do he was much more enthusiastic about the idea than I ever expected. He's already using the trailer as "his" cargo trailer, that is when we're not towing our Havanese around.

Here's a pic of him taking the trailer out for the first time - to the post office - with Pepper (Havanese) along for the ride.

Again, thank you everyone!