TW's are sold in Belgium right? Is the tw125 seat the same? If all else fails get a tank bag to cover up the gap between tank and seat. You will never sit that far forward to need the seat horn.
Example: Or offer it up here in trade. Many folks are shifting to xt350 and or other tanks that sort of need a shorter seat in the front. Good luck and by all means let us know your solution. Photos will elicit all kinds of input (some actually positive). Cheers, Tom