I was just at the Touratech Adventure Travel meeting in Germany (May 8-10) and they unveiled their new (nominal) 20 litre add-on tank.
I say "nominal" because it takes 17.5 litres with a bit of a air cap, but you could fill to the brim and get just over 19 litre. Weighs 8kg I believe. Comes in various coulours and I believe can be painted (original colour is a sort-of matt black). Euro price is 899 unpainted and Euro 1199 for various colourd/painted options. Not sure how easy to paint if you want to do yourself or by 3rd party.
Note that these are European prices and thus only in the German catalog.
I have some better pix but altho I thought within the forum specs it won't accept them Dunno why. It swaps on for the side panels and the piping at the bottom drains to the rear tank with integral fuel pump. You could switch either side leg off and just use the rear (but I think that option not so logical). However it drains from the highest points first, so it empties before the rear tank anyhow.
So is simple and better for bike handling that way (lower centre gravity as you use fuel etc).
Is made of a special thermoplastic that one could repair in the field if cracked: but plastic is supposed to be tough and storng. It doesn't have internal baffles but due to design shouldn't be necessary. Is rotomoulded.
It bolts onto bike and I think it could be swapped on in under a hour. Can taken off too if you don't need it.
It has a drain from the fill cap to inside so any water is drained, or if fuel comes out by vent (if you try to squeeze in until the very brim for instance).
An additional bonus is that combined with rear panniers it protects your legs if you fall over with bike on top of you.
They've made a similar one before I believe: for the 650. So is not a new project for them. A sister venture of Touratech did it: www.tt-3d.com. They do plastics related work for other people too: seems very good little company.
So this makes the F800GS a very real option to compare wrt long distance enduro-style touring if you don't want a 1200GSA.