Thanks everyone for your comments!
Yesterday I bought a 4'X8' Harbor Freight trailer with the larger 12" tires. It was on sale for $300, came in three boxes, and obviously is "some assembly required." The rear axle of the shifter kart won't fit between the fenders but the front axle will; I'll have maybe 1/3 of the kart behind the fenders or so. Given the very light weights involved I'm not too worried about tongue weight. I'll be towing with a Tacoma.
I'm stoked because of two features. Firstly, the trailer folds in half and then rolls around on casters for easy storage inside a garage. Since I've had two car trailers stolen, inside storage is a plus. Also, it puts fewer demands on space as I start to shop for houses.
Second, the trailer bed tilts for easy loading. Well, we'll see. But hopefully I can get the kart on it without ramps. Rolling the kart onto the car trailer by myself when I didn't have an extra set of hands was a major PITA.
Good idea about welding the trailer together. I do have a little welder although my skill level is "burns metal well." It's kindofa shame to ruin the paint but I don't like the idea of the trailer vibrating apart.
Anyway, once I get it assembled and the kart on it I will post pictures. I like the overall utility this offers. The Tacoma might one day be replaced by a wagon (Mazda 3 or Outback, etc.) and this little trailer would make that viable.
David