Mine is 5.8 but I carry 2 x plastic cans (non approved for fuel tut tut) ratchet strapped to the front of the console, never seem to move in any conditions and they are completely out of the way as well as providing a bit of ballast in the nose - hold about 60L total. I carry one of the "shake up and down syphon pumps" on the end of about 4 foot of hose, which is 100% reliable if a little slow for refuelling, takes about 5 min to empty one of the cans I suppose, but a lot less chance of spillage than a funnel which is important to me as the tank is in the console and I wouldn't wish to have lots of fuel sloshing round under my butt!
I always keep my cans full at all times. When my father was learning to fly many years ago, his grizzled old instructor told him 3 things that were useless to you as a pilot:
- Runway behind you
- Height above you
- Fuel in the bowser
Only one of those applies to a RIB but it seems like a good rule of thumb