My first self-designed functional 3D print is a replacement battery cover for a LED fake-candle that my daughter uses as a night-light.
I measured the original cover (we have three of the candles) using a newly-purchased micrometer and tried to re-create it in OpenSCAD. I skipped the screw-hole that is for securing the cover as we don't use that.
I sliced it using Cura and printed it using PETG on our office 3D printer, a Ender 3. Print time was about an hour.
To my amazement, my first take fits snugly!
I've uploaded the OpenSCAD source here: batterycover.scad. It's covered under the terms of Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0).