Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4501995.

Yet another one. Remodeled from scratch so it can be printed with 100% walls, no gaps, no tiny extrusion, better for fast printing, etc. Edit 1: Further tweaked the design to reduce the gaps between walls Edit 2020-07-18: Changed the design to rounded edges, based on feedback.