I removed the foam and didn’t put it back. Any type of foam will eventually take some water and create a moist atmosphere in the tank. Even the expensive hard PU foam takes on some water. I rather use plastic (Coca Cola) bottles or cubitainers like Hartley uses on new boats. Bottles are cheaper though. Even if you decide to re-use the foam, why not take the opportunity and relocate it to a more convenient position inside the tank and create more storage space? Or replace it with a foam board (with the same volume of course) that is glued against the entire ceiling of the tank, creating more convenient storage space. Or do as I did and replace the foam with two waterproof plastic barrels, the sort that is generally sold to store pet food. Not only do they serve as safety buoyancy, should the tank spring a leak, but they also provide safe and dry storage space for sensitive equipment (camera, cell phone, etc), spare parts, a sleeping bag and/or dry clothes. Obviously, while racing I empty the barrels to minimize weight. The buoyancy isn’t much different between filled and empty barrels. Though the barrels are not held in any position, they don’t move around much.
You need a buoyant material in the tank for safety but there is no reason to fix it in a position unless you want it fixed, or fix it in the current position (unless you want it there). Anything that floats and does not take on water will do the job, closed cell foam, bottles, barrels. Just make sure you have sufficient volume as specified by the class rules (MK1: rule 34.4a – 40 liter minimum).