I’ve towed using one of the 4×8 foot Harbor Freight open trailers for years, and think I’ve arrived at a pretty good scheme that is as light as possible. I carry two karts that I load across the trailer (one facing toward the passenger side and the other facing toward the driver side) one behind the other. The trailer has two 4″x4″x 8ft boards attached lengthwise (spaced apart similar to the way a kart sits on a stand), and the karts rest on their frame rails on top of these carpet-covered beams. I’ve installed eye bolts on the trailer rails located to allow for tie-downs to each kart axle (2 per kart) and tie-down to the frame rails near the front wheels (also 2 per kart). Tie-downs can be whatever you’re comfortable with and confident that they won’t loosen in transit.
I’ve used this setup for six years now, and I’ve never had any issues. The tow vehicle punches a big enough hole in the air to protect the karts, so road grime hasn’t been a problem.
I recently used this rig to tow 2000 miles cross-country in preparation for moving back east, and it worked fine the whole way.