Fuel return line design obstacle

My opinions (based on some experince)

Vent

Do not drill the tank filler cap. I had an aircraft that had this feature - as you already know this leads to liquid fuel running down the side of the aircraft.

If possible, have your vent line/hose rise to the highest point in the aircraft and or a "break tank" before exiting.

Fuel Return 

If you can gain access with a drill,  why not have the return line in a similar position as your existing vent point on the tank filler tube? (be sure to have the vent above the return line) If you use a threaded spigot (example shown) you should be able to put in in place, using a wire trace, through the hole you have drilled.  Hold it with long nose pliers and tighten the nut on the outside. Use good quality fuel compatible sealant when installing.

Fuel Return Line Restrictor.

Rotax recommend a #35 jet. (On my installation this is too large). You can purchase the jets from Mikuni (VM22/210 Mikuni Pilot Jets - various sizes). The restrictor jet and the boost pump have a relationship  - Rotax recomend Max 5 psi RV recomend Max 7 psi. 

Fuel Lines

I would strongly advise against aluminium fuel lines - Cost, many more failure points than "rubber" hose line, much more difficult & time consuming to install and potential for fatigue cracking. Use good quality fuel lines eg Gates Barricade secured with fuel injection clamps (not the serrated hose clamps shown in your photos) or similar and don't do them up too tight. Just replace every 5 years - easy!

Thanks a lot for the reply, that was very useful. I just noticed on one of the pictures that there actually seems to be another port right behind the vent port that I had never noticed before, you can tell by the hose clamp sticking out slightly. I should check it out before drilling anything, but I'll definitely keep the suggestion in mind in case it's unsable. It's quite hard working on an airplane that you don't always have access to and rely mostly on pictures haha. You are right about the aluminum lines, what I will probably do is use mostly rubber lines but use at least one hardline near the firewall since there are a few tight 45° bends that need to be made, and P clamps are sometimes impossible to install due to very thin wood.

The return line should originate at the highest point of the fuel system near the carburettors but it is OK for it to enter the tank at the bottom. The hydrostatic pressure is not a problem.

You mention the possibility of teeing into a purge line - I think by this you mean the water drain line? Exactly this solution is used on many Europa aircraft.

Personally I think aluminium fuel lines are much better but they must be well secured. I hate the long flexible hoses on the Europa and they are extremely difficult to inspect or replace.

Good to know it has been done and works, I definitely meant the water drain lines. At least now I know I have a backup solution that doesn't involve modifying the fuel tank !

I agree, hydrostatic pressure is not a problem. My Kitfox model 7 is a high wing with fuel tanks in the wings, and the return line goes into the side of a header tank behind the seats. This gives a head pressure on the return port of about 2.5 feet, which is no problem to thousands of Kitfoxes out there.