Hi Callum, have you looked into these? I have the full set and they are really heavy, a lot of structure and not a great deal of space, I assume you have the top box so I guess you know what I mean. If I were after luggage I'd look at lighter gear. Unless I've already got the brackets, but then I'd still consider it. Keep your eyes on eBay/gumtree
Hi Calum and welcome to the forum. Which Tiger do you have? I have these Expedition panniers fitted to mine - love the look and very practical as they load from the top. Make the bike wider though as the offside one sits out from the exhaust. http://www.triumphmotorcycles.co.uk/accessories/luggage/2014/aluminium-pannier-kit-silver-a9500600 Givi do similar ones I believe, which may be slightly cheaper.
Hi I`ve got a set of these, they`re cheep and light and hold a lot. SW Motech Dakar Motorcycle Luggage Panniers - Triumph Tiger 800 XC|XCx|XR|XCa £280 on ebay.