on a cassette there are ramps on the cogs so it will shift up smoothly these ramps will also try to lift the chain up the block when running a single speed thats why a singlespeed cog is just plain its the spring tension on the rear mech that stops the chain jumping gears all the time, you could try it though but i wouldnt expect it to be as smooth as a bespoke cog.
Get a couple of old cassettes and smash the holding pins out of them and remove the spacers. Pick the cog you want and fit that with the spacers and the lockring cog. Take out the top jockey wheel of the rear mech and put a short piece of cable with the knob on that is normally in the gear lever at the barrel adjuster for fine tuning alignment.
Jobs a good un.
UPVC waste pipe (32mm IIRC) is good as spacers.
A Shimano DX BMX cog is better than one out of a cassette as it is unramped and slightly thicker. I use Surly ones as they are a lot thicker and less likely to damage the cassette body.