This is a simple lid for 1-L Imhoff cones. We use Imhoff cones to maintain hatched brine shrimps over the day for feeding anemones. The lif has a slot to fit an airline tube for aeration  and prevents splashing or contamination. The bottom is shaped so that water drops can drip back down.

It can easily be printed upside-down in PETG with 0.2-mm layer height, three or more perimeters and 25% infill. No supports are necessary.