To find a generic cone for any hub you need more than the length. You typically can't go larger in diameter without altering the dust cap, or smaller without fashioning some sort of seal (o-ring, silicone seal). In addition, from looking at Shimano's diagram
http://techdocs.shimano.com/media/te...9830608668.pdf it appears the hub uses a labrinth dust seal, which makes replacement even more problematic.
Overall it's a frustrating situation for someone who does not want or can't afford to replace entire units for want of one part. 20 years or more years ago it was easier to find compatible parts or even make do with something close, but that is much more difficult now. There seems to be little interest by the manufacturers of many items to maintain any kind of cross generation compatibility, to have parts similar in specs to others, or to maintain a repair parts inventory.