My brother recently bought a coker, and I’d like to ride with him. But he has a job, and thus money. I’m a poor student, but I have access to welding equipment, and metal. I also have a spare hub, seat, seatpost, cranks and pedals lying about. So I figured I’d construct myself a 29 inch frame, get a 28 inch rim off an old bike, and build myself a nice 29’er. Now, first things first, I need a frame to bolt all my bits and pieces to. I’ve taken a few quick looks at my trials uni, and at the coker frame. Ofcourse, there are two obvious differences. For one, the coker has a curved crown( /\ ), and the trials has a flat crown ( |-| ). Are there any advantages/disadvantages to either design? I also noticed both uni’s have a slight widening at the base of the pipe where the seatpost goes in, why is that? The rest seems pretty straightforward, and I should be able to quickly assemble it, and start practicing for the 10km in Unicon 
You might be able to buy a Coker (or 29") frame pretty cheap from someone who has upgraded, or otherwise from anyone trying to get rid of one. This might be worth looking into unless you really want to get your hands dirty and build your own.
Crown shape is a more a matter of manufacturing method than much else, unless you’re getting into high performance. Build whatever will hold up and be rigid. I’m not sure about the widening at the base of the unicrown forks, but the think to keep in mind is you want your seat tube to be the right size size for your seap post. You can’t just narrow it down for an inch or two where the clamp is (like on Miyata giraffes), or you’ll create a weak spot.
The easiest approach may be to build from an old bike fork. That will cover the crown part, and you’ll just have to figure out the bearing attachment (and maybe length), and possibly modify to fit the right seat post.
I think you can buy a new 29" Sun for around $90. I don’t know if you could build one much more cheaply than that.
Then you can upgrade it as you go. I rode one for several months. It’s a pretty fun ride - and fast with 102s.
look at the torker ax unicycle. It comes in 29’’ and they’re super light. And about the crown flat crown or rounded doesn’t matter on a 29er unless you want to go one footed then make it rounded. but also keep in mind how big of a tire you want to fit. Especially if you’re going to use it for muni.
Its not out yet.