Based on some excellent suggestions from others, I bought some lightweight 7mm corrugated conduit (used to tidy up wires in car engine bays) with clamps to suit.

My wife assisted in running a lightweight string through the conduit, I then installed it using some home made 0.025" brackets and taped the ends of the string to the root rib and tip spar.  Now, if I decide to install tip lights, just a matter of taping the wires to the string and pulling them…

Slotted in the Savannah tank last night.  Fits like a dream!  I made up a couple of brackets fitted to the ribs to support the rear of the tank, clecoed on the front brackets that bolt to the tank, and ran fuel line through grommets in the tank channel and rib #1.

The tank feels well supported and pretty much immovable, and the top surface looks like it will be very close to the top skin.  I'll run a straight edge over the ribs and pack out the top of the tank with cork strips or…