If you want to be really cheap, you can also collect a bunch of trashed cassettes, break the pins that hold them together, and harvest the spacers (you will need two or three to get enough). Get a SS cog (or cog from a separated cassette that's not too chewed up) of the size you want, arrange the spacers and cog on the freehub body so the cog is in line with the crank, lock down the lockring and you're ready to go.
As long as you don't have vertical dropouts, that is...