Build one, or pay someone to do it for you., need a 36 hole track hub,
because you need the left hand threaded lock-ring to keep the sprocket on
when you submit counterforces on the pedals to stop.

I have retensioned spokes, and shifted axle spacers
to turn a 5 speed freewheel hub into a single speed wheel ,
but that is a single freewheel gear, fixed gears need the lockring.