Try mounting smallish cans well FORWARD!
Either in front of the saddlebags, hanging off the front of the saddlebagbag, or all the way forward, and - in keeping with the rat-bike theme - in a pair of daypacks tied together and slung over the gas tank.\
I've done it myself, and it works well. Big thing most people miss is that there is a vast empty area up at the front of the bike that can take the heavy gear quite well, and moving the weight forward greatly improves the bike's handling at the same time.
Just don't get carried away with the total quantity of fuel.
My bike has a 40 litre tank - that's 10 US gallons - and it's more than needed virtually anywhere in the world - in fact I'm kinda looking for the 35 litre version of the same tank - (Heinrich for an R80G/S if anybody knows of one on the loose!)
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