ORIGINAL: Jimmbbo
Refill thru the vent using the field box pump... Shouldn't be an issue, but will require plumbing to do so (Maybe a T in the pump inlet line with a fuel dot at the end that is removed and placed in the gallon jug to refuel the fuel cells in the field box... Nice and clean!
The tanks can actually be oriented in any manner, so long as the clunk is on the bottom and the vent is at the top. They could be set in the box with the caps pointed up just like a fuel jug... run the clunk line a little longer than normal, and cut the vent short so it barely penetrates the cap... or run it all the way into the bubble and you'll just lose the top 1/2" or so of the tank's capacity.
To refill, it's just like when filling your plane (with a header tank)... you fill through the carb line.
Take another look at your drawing (which is otherwise correct)... if you fill through the top vent, then the fuel will immediately be pushed into the clunk, which will feed the next tank's vent, be pushed out the clunk, and so forth until it comes out the final clunk line.
Instead, you simply reverse the pump and refill the tanks in exactly the same manner as if you were draining your planes tank at the end of the day. No plumbing changes needed.