That fuel cap needs to completely seal off the fuel from external breathing or moisture ingress. Diesel fuel is highly hygroscopic. It absorbs moisture out of the air at an alarming rate if your tank has access to it due to daily cycle of high and low humidity in the air, due to adiabatic process of warm air holding more moisture, higher dew point, and cold air holding less moisture, lower dew point, and condensation inside the empty portion of this fuel tank.
... "at an alarming rate"... geez? Diesel fuel isn't "hygroscopic" enough to pull enough moisture(water) from the air to make enough "free" water. Free water is the stuff that will cause the damage you speak of, but that's what the water separator and the "idiot" light to warn you is for. Even the "water filter" isn't enough to remove "dissolved water" or emulsion (water filter doesn't remove dissolved water, only stops the "free water" to a point). Adding fuel treatments that are designed to remove emulsified water (d-emulsifiers) do more harm then good, as they will cause the accumulation of "free water". Accumulate enough free water in the tank and you'll have issues.
Typically, diesel at room temperature will contain about 200ppm or 0.02% dissolved water, right from the refinery. I'd be more concerned with microbiological contamination, then air humidity!
