I used to have one on my HP2E......It would hold a five litre fuel can in each of the lobes aswell as expanding to hold the rest of your gear! I bought mine second hand off here for £190 or something and then they went to 'silly money'........!
Don't remember it being very waterproof though and I replaced it with the 29/35 litre Touratech Zega pannier system with the stainless luggage rack!
I've heard good reports with the 'Giant Loop' my German friends use on their KTM's
FP.

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Me and my mates have just used a simpel 50l waterproof bag from Heingerich for 20euros strapped over the back seat that seems to fit everry thing in for one week in the dollimites camping withaut erfechting the handling in anway licke thouse sarga boxes can as I have experanced ;-)