I wanted to share it here. I bought some 4cm,2gr each Styrofoams. They perfectly fit in without bending or pushing. 9x2 plus 1 bonus. They are locked in there. Was a bit hard to push them in. Now they move a little inside but they don’t go out. I hope my boat will never take water nor have a crash but it is better to be safe than sorry. I don’t think ballon idea makes sense.