andy,seriously mate,you want to look at making some 2 in one back rest/carriers for gts's. theres a choice of..wait for it .. 2 backrests and 1 carrier which you need to buy before you can have a backrest.. ludicrous!
well over £250 if you want to use a gts 2 up with luggage and thats before you even look at a front carrier..
massive gap in the market for someone with a bit of no how..