When I was learning on my 20" Torker and biking 5 miles to the Amtrak station, I just used a couple bungee cords to secure the Torker to my rack. The bungee cords were long (I think one was 1.5’ and one was 2.5’, unstretched), and I wrapped them around lots of places on the uni.
I have a homemade post toward the back of my rack, which helped. It’s about 3" high, and made by drilling a hole in the platform on my Planet Bike rack, and using a bolt, nut, metal washers, garden hose washers, and a short bit of copper pipe. What you’d see looking at it top to bottom would be bolt head, (rubber) hose washer (for noise reduction/shock absorption), three inches of copper pipe (sheathing the bolt), metal washer, rack platform, metal washer, rubber washer, nut. Of course, I used loctite to keep it fastened. It is heavy because of the 1/2" diameter 3.5" steel bolt, but its utility is worth it. In case you’re wondering, I worked at a hardware store, so these parts were convenient for me. Not everyone has a 3" length of copper pipe laying around.
So I put the wheel on top of the rack and put the frame against the post, then bungee corded the heck out of it. The frame would still travel a little while riding, but it was manageable.
I’ve carried another bike on my bike before, when I had a milkcrate (non-stolen!) on my rack. I got as many looks with a bike on a bike as I do on a uni. I guess people aren’t used to seeing bike-on-bike action! No one in front of me could see the uni when it was on my bike, though. Anyway, I think I would be able to just barely manage my KH24 if I had the milkcrate on my rack, by setting the wheel on top of the crate horizontally with a crank suspended in the crate, and the frame pointing behind me. You could take the seatpost and saddle off, too, if you have such a milkcrate to put it in. You could put it in saddle-side down, then it could stick up through the spokes if the spokes aren’t too close together.
Again, bungee the heck out of it–hook the bungee to the rack (or to itself after going around the rack or the wheel), then stretch and wrap, stretch and wrap, stretch and wrap…and hook. Wiggle your uni throughout to see where it’s loose, and try to bungee it down at that point. It’s not fun to have a massive object swaying on the back of your bike, especially if its center of gravity is high.
For any long distance (greater than 5 miles), I’d have to use my bike trailer (designed to carry a dog). Then it’d probably be easy up to a 29", but a 36" uni might require a custom trailer. But at that point, you’d be hauling a uni built for speed and distance, so that’d only make sense if you wanted both vehicles at your destination. And I’m done. :o