I always intended to add a stem to the bulkhead fitting inside reaching upward slightly to prevent the tank draining completely in the event the transfer line is ever severed. I also wanted the ability to see the fuel transfer in progress. The stem turned out to be a necessity in getting the bulkhead fitting to the hole.
Voila! Bulkhead fitting installed. This fitting also came from Maple. $20 shipped.
One of the problems I encountered on my little test ride across Texas was that fuel would not transfer from aux cell to main tank unless the cap was open. Venting problem. I have learned the tank can and usually does operate under some pressure. Pressurization is not requisite but rather a byproduct of the fuel pressure regulator constantly shooting fuel from the fuel rail back into the tank and the fact that the oem cap is equipped with a flapper valve on the vent line to let air in but not out and is designed to close in a tip-over. Here's the vent line at the filler, the small opening between 4 and 5 o'clock. The larger port around 10o'clock is the overflow.