You can do this yourself. Either buy a 2nd hand seat or modify your original. The cover is only held on with staples, so just buy a staple gun. Remove the staples, cut the foam down and staple the cover back in place.
Or the perfect Christmas pressie, send the seat off to Tony Archer's, get him to cut it down and insert a gel pad, to make up for the reduced foam. Pukka job then if you don't fancy doing it yourself
http://www.tonyarcher.co.uk/