My room aint exactly the cleanest place in my house and i have issues with ants and the occasional baby roaches falling into my tank which making me clean my tank and change water almost daily.
The sample pictures were printed using PLA as its my current on hand filament but its better to use Food safe filaments or TPU for a more snug fit.
Printed with supports using the Prusa slicer at .28 layer height as its an internal piece that wont need to be visually pleasing.
Some post processing might be needed if the shape of your watertank got bent differently from what my watertank shaped like ( I have included the top scan of my watertank incase you are curious if we have an identical bend/deformation)
The author hasn't provided the model origin yet.