If you own this https://www.muziker.sk/nuova-rade-flexible-water-tank-rectangular-100l you may need only one opening to fill and empty water tank. This lid can seal the inlet water opening.
Just put it there instead of water pipe. Maybe some gummy ring seal can be usefull.
Print instructions:
infill: 100%
perimeters: 3 or 5
material: PETG
The author marked this model as their own original creation.