Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:5089972.

When printed in vase mode, this cap will fit over the bottom of a 3.5" aquarium intake filter to prevent air bubbles from a bubbler placed below being pulled into the hang-on-back filter.

Specifically, this exactly fits when using the Aquarium Co-op Large Pre Filter Sponge with a Top Fin® Silenstream™ PF40 filter as comes in this 37 Gallon Top Fin® Essentials Aquarium Starter Kit.

If you have a different size filter and would like to make changes, the FreeCAD project file is included so you can make changes.