I built one for mine, it is lower than most street bikes but higher than a kneeler..
When I decided to build it was a Wed night..
Thursday, I had made a wire template to bend tubes and took it to the muffler shop.
Friday I got the metal back, I may have started Friday, I know I flogged all day Saturday & into the night..
Sunday it was on the vintage dirt track running w/o other bikes.. I was the monkey!!
Will try to find some pics..
Basically a platform with a small wheel, I used a Cushman wheel first, then built a hub to run a 50 Mini-trail wheel. (sorta close to first pic)
The rig comes off in under 15 minutes!
In Europe 10 or 15 years back they were popular, in street frames with smaller auto wheels and leading link front.. works way better.. what I am saying here is mostly street frame with smaller wheels, is a lot easier to build than a kneeler..
Also I myself do not think the kneeler height is good for the street as they already cant see you on a bike 2wice as high..
It was great fun..