If you do have a cassette style wheel, you can make it fixed by getting the freehub welded. I have one of these and would be willing to sell it(welded freehub body). Be forewarned that this will not work well with a standard quick release axle. Trust me I know, but it MAY work if you used a solid axle with bolts like a regular fixed gear bike. I had one welded up for testing and had some problems with it while using a quick release so I abandoned the fixed and bought a bmx freewheel for my old threaded wheel which used a 7 spd freewheel. Let me know if this is something you would be interested in. I can also take pictures if anyone wants to see how it looks or explain in more detail the problems I ran into.