Changing standard ratio's on shimano 10spd

[Chris NewbyFraser]

Like many riders, I seldom use he 11 tooth cog on my Trance. In order to maximize the use of the cassette, I am wondering if it would be feasible to source cogs (buying up used complete cassettes, or new) and then 'build up a closer ratio cassette.

I can handle bigger changes on the range from 34 down but would like to aim for: 13-14-15-16-17 at the bottom end. I need the 13 as it is the smallest size, I think, that has the threading to lock the cassette together. The spacers of these cogs may be a fiddle though.

Any thoughts?