I haven’t tried this for any great length of time yet, but I just thought of a method of training for general unicycling balance and particularly still stands that seems pretty effective. There’s a spot at my house where a shortish bit of rope (for my hammock) hangs down. I set myself up underneath the rope and it’s just at the right height so that I can comfortably use it to correct my balance.
It’s good because you can do little still stands and work on using the rope less and less, and it’s not at all tiring. A few minutes ago I was sitting in a wheel walking position and balancing on the spot.
Give it a try!