If this is serious, then the proper way to go about it is to heat up the fork crown and remove the existing steertube. Buy a new steertube from any of the numerous sources and braze in a new one that is the correct length. This should be done by a frame builder or someone with both the proper tooling and brazing experiance. Forks are not where to take short cuts unless you are really wanting to spend some quality time with your dentist.

Here is a new steertube. It's $10. 1" FORK STEER 240mm :: TUBES STEEL :: Nova Cycles Supply Inc.