I just used a kid's trailer, un-modified. I did replace the steel rims with alloy, but only because I'd just got into wheel building and wanted the practice. When the dog grew old and died, I sold the trailer as a kids trailer to someone who wanted it for... kids.
Incidentally, my dog loved getting out with the family on the bikes. Mind you, we stuck to bike paths though that was more due to having young kids on bikes than having the dog as well. I've a photo somewhere, that I can't find unfortunately, showing me on a hybrid, my daughter hooked behind on a tag-along bike and hooked to that, the dog trailer
