This egg always stand vertically once you release it. It contains at the bottom a 5/8" or 16mm metal bearing ball that shifts its center of gravity close to its wide bottom.
I printed it in its vertical orientation with material support only from the printing bed (that is, no material support inside the egg). When it completed to print the internal cylinder I paused the printing, dropped the ball in, and added some glue from a hot gun (epoxy or any similar glue should also be fine), and then un-pause the printing.
When the printing completed, I cleaned up the bottom with an X-Acto knife and sand foam sponge to round the bottom. Then balanced it by placing on a flat and smooth surface, observing the orientation and sanding slightly the side of the bottom that is too high (the egg tilts away from the high side().
PLA, 0.2mm, material cooling fan on, 30mm/s, support from printing bed only (not inside the egg).
If you want to tweak the design or dimensions load the .scad file and generate the STL using OpenScad (free, open source, runs on all major OS's). I tried to mark it as customizable but Thingieverse timeouts too early.