Dave Gray showed up towing a trailer, loaded with a unicycle, two large boxes containing everything but the frame for a Raliegh Twenty, and a large plastic tote containing who knows what. The aluminum trailer, built without welds, wasn't loaded anywhere near its capacity. These trailers, and other utilitarian HPVs are made in Ames Iowa by Bikes At Work.
Dave's off road Unicycle was made from an MTB fork. It will accept 24" 26" and 29" tires. (Fat tires on 700c rims are refered to as 29" now.)
Cantilever studs are positioned so that, with a 29" wheel, brakes can be fitted. Sounds like an instant face plant, but apparently the really insane off-roaders go down hills so steep that (careful) braking is needed.