There's a lot of merit in Sheppo's idea, but inspectibility does seem like it would be more difficult.
Maybe the whole issue of back-up flotation is another one of those things where you have to imagine the circumstances of the need.
What would you have done to damage the float enough to rip it open and turn the ping pong balls loose? Can you imagine your Baby floating happily on it's floats but inverted like an overended catamaran?
I found ping pong balls on the web for a dime each which would bring the flotation bill to under $3,700. My thought was that the protection you are seeking is for the plane tethered to a mooring while you go for lunch, not as an aid to locating the wreck after an unfortunate meeting of float and large foreign object.
