I built my own for under $50. Bought the Nimbus foot plates from unicycle.com and I got a 20" free-hub w/ a 14mm axle (front wheel of a bmx bike). Built the thing from scratch. I got everything else free b/c I work at a bike shop.
It is VERY hard to stay on. I’m SLIGHTLY improving…able to go 6 or 7 feet before messing up. All it takes is practice and dedication. 20" is just fine, and a pretty standard bc/uni wheel size.
Put your hands out at a 45 degree angle upwards. Make sure your knees are not locked. You want to be crouching ever so slightly. When mounting put all your weight on the foot thats pedaling up till the second you put it on the plate, then put weight on the foot thats on the non-pedaling foot. Bow your knees out to stop any tire skims. Keep your feet flat. Look forward not down. If you start to slip back piviot one knee in and rub the tire with your leg to bring yourself forward again, Only a quick touch though. Tense your legs up to keep the wheel centerd between your legs, this way you dont have to bow your legs as much to avoid the tire. Try to run a rather slick tire. Purly slick can be bad, if oyur going fast you cant catch yourself from sliping back nearly as good as with a slightly rough sidewall. You dont want knobs as much as you want just a rough tread pattern, Like a hookworms texture or a primo comet’s tread.
If you run a mod rim, Use a echo supa trial tire, Cheapest and best mod tire for bc. If your run a bmx use a 20g if you want wide and bounce, if you want high psi and slim run a primo comet or maxxis miracle tire. If you run a 24" use a hookworm or halo twin rail. Same if you run a 26".
seeing this erected from the grave, how about making a hunter style frame to go over the bc like your first design. and inbetween the frame on the side, you could have little leg pads like what they have for the back of a snowboard binding. and then just get those magura brake mounts that you can weld on so that you can use the hs-33 drag knob in case you want to control speed but not fully brake.