Hi Xmax, thanks for your good wishes.
I don't think having a filling restriction with floating device will provide either value for money, or peace of mind. The difference in head height of water between the top of the tank and the filler cap will make very little difference in regard to the pressure on the sides or top of the tank. Your problem I think is that your boat most likely had a tank with faulty construction which combined with rapid movement of water, or sloshing within the tank while it was only partially filled, most likely caused the damage. Keep in mind that one atmosphere pressure will support a column of water some 32 feet in height, so the head height of your filler cap would make a difference in pressure of little more than a puff of wind. On the other hand, slamming into a heavy sea will cause the contents of the tank to accelerate forwards and upwards at high speed against the top of the tank, and that will give you more grief than the very slight increase in tank pressure caused by filling to a level within the height of the filling line. In such weather conditions you would most likely be better off filling the tank until it overflows, and not using any water from it until the weather had improved.
However, the foregoing does assume that you only fill using an ordinary hose poked into the filling connection, and not by some method involving coupling the filling hose to a permanently fitted connection that would not allow any excess water to simply overflow. In such instance the hydraulic effect would overcome the strength of the tank in no time at all.