If you plan on taking off the cassette often, it wouldn't be the worst investment to buy a real one, but as other people have noted vice grips and an old chain work fairly well. I think you can get one at Nashbar for $8.
I used a home made one for quite some time, and it worked, but I sometimes had a harder time getting leverage than with a real one. I was also a little concerned about accidentally damaging the teeth of the cassette with the home made job. That would, of course, be a bigger deal if I could afford nice cassettes.