If you want to do this on the cheap, it might be better to just get some spacers and take off all the cogs except the one you decide to use, keeping in mind chainline. If you were to put a single bmx-type freewheel on I could see some probably CL issues arising, mainly due to the dropout spacing in the rear. When I was younger and made my first 6 speed to SS conversion, I used spacers and it worked well.