I made myself a normally sized unicycle a month ago and once I knew how ride it, I asked myself: why not make a tall unicycle for it to go join it's fellow tall bike?
This design was totally improvised as I advanced in the construction process but it turned out quite well (even if it looks a little sketchy).
I used 'leftover' parts from bicycles I used in previous projects so this cost me absolutely nothing!

Step 1: Materials

What I used:

Tools:

Grinder
Arc welder
Clamps, screwdrivers and so on...

Materials:

Rear half of an adult size bike frame
Children sized bike frame
26'' rear wheel
A length of metal tubing

Step 2: Blocking Freewheel Mechanism

Obviously, for a uncycle you want to get rid of that freewheel mechanism because you want to be able to pedal backwards. I'd never done this before and I din't look up how to do it so I just improvised an did it my own way. I unscrewed the little metal plate/cover that held the entire mechanism down, took out the tiny ball bearings that I discovered under this plate and welded the freewheel assembly with all the gears on it to the actual hub of the wheel. For this reason, you want to find a rear wheel with a steel hub because if it's aluminium there no arc welding...
Once the freewheel is blocked, you can easily unscrew the first gears and then remove the other ones.

Step 3: Starting to Make the Frame

I used my rear half frame part plus a length of metal tubing to make the base of the unicycle frame. I nothched all the tubes using the grinder to be able to weld the bottom bracket in. Note that the bottom bracket you see in the pics is not the one that will be later welded in, it's just used as a model to determine dimensions and the curvature of the notches.

Step 4: Welding in the Top Part of the Frame

The top part of the frame is the bottom bracket with the seat tube from the original bike still welded to it. Now at this moment it's critical to align the top of the seat tube, the bottom bracket and the axle of the wheel on a single line BEFORE welding.

Step 5: Making the Chain Drive (1)

The two gears you use must have the same number of teeth. Usually in the casette (back sprocket), there is a gear of the same size than the smallest chain ring. So I took off the two larger chain rings from the crankset by drilling off the rivets that held the chain rings together. I kept only the the largest gear from the cassette which had 28 teath just like the smallest chain ring.
I then welded the the large gear from the cassette to the rear wheel.

Step 6: Chain Drive (2)

I cut the chain to the right lenght and reassebled it. However I realised that a tightening mechanism was needed to make the chain tensioned enough so I used the same system than on BMX's. The pictures speak for themselves.