Make sure to print it out of something food-safe like PETG.

I did it out of white ABS on a school printer, sanded it down, then did several layers of paint on the external piece, finally finishing with a thick layer of PVA glue as a pseudo-varnish.

It performs pretty well at keeping a hot drink warm, and looks nice as well.

Apologies for the lack of photo on the mug, I don't have access to it at the moment.