As I understand it, a lot of people used to use slosh products, and they caused problems later on. Fuel tanks leaks are a pain. I had one, but could reach the area through the filler cap. Two years later, no leak.
Search around on this site for threads on tank leaks. There are ways of putting a VERY slight vacuum in the tank and drawing in a thin sealant. Beyond that, bite the bullet and cut a hole in the back. I know that isn't what you want to hear, but it is better to get it right now.
Bob Kelly