I was surprised not to find a sew-on egg template on Makerworld, so I designed one that perfectly resembles a real egg. It takes a while to print, but that's to ensure it's perfectly smooth.

After printing, I recommend lightly sanding it to make it even smoother before gluing it on. Be careful, there's a specific orientation; you'll quickly notice it. Press firmly to glue it, then sand it again once it's in place.