Do you have any other old equpment lying around? A VCR perhaps? If so, it should be able to take composite RCA in (the red, white and yellow plugs), and output via coax on channel 2, 3 or 4. The old RF modulator trick. If your TV is really old, you also might need a 75 ohm coax-to-wire transformer.

So:

Xbox -> VCR -> TV