Take a sled and add a pivot as far away from the blade as you want your radius. (riving knife and guards can remain in place)
As preparation rough cut your piece on the band-saw.
Then put your piece on the sled and attach it to the pivot. Clamp it down to avoid the piece rotating while you are cutting and make a cut. Unclamp, rotate, clamp and cut again; rinse and repeat until it is round.
To get it perfectly round while not rotating the wood against the blade is nearly impossible. The last passes would require very accurate turns.
Instead if you have a motor and some shaft and bearings handy you can attach you can attach a pully to the wheel and work on it like a lathe.
The main advantage of this last step is that you are turning the wheel on its own axle so even if you make a mistake when mounting the bearings/putting it on the shaft you can true out the wobble.