Hi Urs,
I do agree that you want caps that seal properly, but I also find it somewhat surprising that you attribute a leaking fuel cap to the accumulation of a gallon of water. If you leave the cap completely off, that results in an opening of approximately 3.5" diameter (maybe a bit more). Not doubting your data, but it would require a 24" column of rain to pass 1 gallon through a 3.5" opening. Please check my math on this.
Have your fuel tanks ever been repaired, or do you have bladders? If, for example, fuel equalization holes in the wing ribs are blocked with sealer during repair or relining, as I understand it water is not free to drain toward the wing sump and a surprising amount of water can accumulate in the wings over time. This could present a real problem irrespective of the condition of your caps.