I was gifting a mug to a student that plays softball, so thought this would be a cool idea. Modeling one, not so much, but user TwizzleStark posted a GREAT model https://www.thingiverse.com/thing:796123 that I pulled into blender and edited by adding a bat handle, a hollowed out center and mouth and IIRC, I filled in some gaps that might have presented problems in leaking. The end result was pretty nice and she loved it.

The mouth is small-ish, but I didn't have a hard time using it. At scale, I believe it holds around 14oz or so. I'm also adding a version without the handle. I printed a scaled down version to test with.