A lot of car valeters use these type of tanks (I used one for years myself) and have the same problem, I've bought tanks from several different places but these black lids always leak. It seems when tight it's tight because the cap is touching the tank, not because the rubber o-ring has sealed - then leaks. The vent in the middle is S shaped and doesn't let water out, just the threads/cap seal.
I currently trad clean, but will be getting a tank again soon, and like the pictured solution of glueing in shut permanently.