http://www.redagascan.com/portfolio/honda-goldwing-gl1800/Across the passenger seat would be the best place.
If you have a passenger, I would look into the gas can made to fit in a GW saddlebag.
Have two of these! put one in each bag on the trip to Alaska (never needed them). Just remember to make sure they are sealed tight, Neither of the cans leaked One how ever did vent fumes, Ideams in that bag needed to be aired out for a while. the other was not a problem at all.