It doesn't matter how much duct tape you use...
It actually went fine. I've now taken 3 rides with cats in the trailer (to the adoption center, return from the adoption center, and a roundtrip to the vet's with one of my own cats). They didn't love it, but it was no worse than the car (based on the quantity and quality of meowing... occasional complaining, no outright panic). I made sure to take only quiet streets and MUPS, and tried to minimize bumps (not easy, since roads are snow-covered right now).