Look for a couple cruisers.. to modify. cheap budget I suspect..?
a hub with a long rear axle, nuts to hold it on the frame,

make, or have a well equipped welding/machine shop do the work..
something like a widened unicycle fork, with a bearing in the end,
to bolt onto the rear end , then tighten it down, the inner race

to be tight around the axle, but have bearings

so it won't come loose but is fee to rotate around the rear hub axis..

Or maybe just get 2 or 3 unicycles, and hold hands.