I decided I can use 3/4" pipe unions (which are three-piece deals; two of which have threaded ends and the third is a nut that keeps the two other parts together. They are beefier than typical pipe so they should be strong enough. Unfortunately, they're about $3.50 each so I'd like to see if I can find them as they are removed from old construction.
I also noted that even on eBay, the base price for a unicycle is $75 with shipping. Unicycles.com has the "cheapest" unicycles which sell for $63 with $12 shipping. On eBay the situation is worse with sellers using exorbitant shipping costs to offset low prices. So far, $75 is pretty much it.