any cheap screw on freewheel wheel set can be re-tensioned to be a single speed .
its actually stronger , as the spoke tensions become more equal, as you take the 'dish'
out since the 1 speed freewheel needs to be further to the right.
and the room needed for more cogs on the Hub/axle is gone.
the cassette spacer singleizers dont take advantge of that,
but is a simple reversal, if you change your mind.