I studied the limited information on the inverted fuel modification at some length. Besides the drawing, I also talked to Van's and did some internet research. I considered not installing the trap door at all but in the end decided to follow the plans. In inverted or knife edge flight the door should slow down the draining of the fuel from the inboard bay with the flop tube but is not meant to stop the flow entirely. I don't really know how long the plane can remain inverted but I suspect it's long enough for me.
The only changes I made to the drawing were to position the door in slightly from the baffle flange and up slightly from the intersection of the rib flange and skin. This was to keep the door away from the proseal. It may leave a very slight gap at the very bottom of the rib hole (see iamge 2 below). However, once the proseal is applied and a fillet created between the rib and skin, any remaining gap should be small. Regarding the hinge pin, I put a 90 degree bend in one end and safety wired it to the rib. I wanted the hinge pin well away from any proseal that might squeeze out between the baffle and rib flange (the edge of the trap door assembly is close enough).
I located and cut a hole in the tank baffle (second bay) for the fuel sender. Rather than returning my unused std access cover for the left tank, I jigged it up in the drill press and cut the fuel sender section out with a fly cutter to create a reinforcement plate for the baffle. All the holes were drilled, the baffle dimpled and the reinforcement ring countersunk. By the way, the two fuel senders are not identical (one is for the left tank and one for the right). I did some head scratching until I realized the units had different part numbers.
