If the tank is the same size as the hole in the fuselage former, then it's difficult to get any foam between the former and the tank (obviously as you have stated). Generally, I just don't use any foam at all in this situation. There is much concern expressed about fuel foaming and in my experience have not found fuel foaming to be that much of a problem. So what I am saying is even though I understand your question, I would avoid hacking out the former just to add foam. What problem are you trying to solve? Is your engine running poorly, etc.? If you are seeing bubbles in the fuel line, for example, the problem could be a hole in the tubing. This is a more common problem than foaming.