RE: usable fuel
That's the point, you don't want the clunk to move to front of the tank, and I've never seen a tank where it could easily do that with out getting stuck. And it getting stuck is a bad thing.
Maybe it could work in some of the really large tanks out there, but even the 12 or 14oz (I forget which) tank I have wedged in to an LT-40 won't do it, and shouldn't do it.
Don't worry about it, just put the tank together with a single clunk just a little bit from the back of the tank and go fly. You won't have any problems from that. If you really want to fly for an hour, put in a bigger tank (like I did in that LT-40 above. I have no idea how long it will go on a tank, but based on fuel left after long flights, I'd say 45min of aggressive flying, over an hour if I just put around).