[Kaderin Triste]

Originally Posted by Biomecha

Ah, the one I have now keeps drinks hot for too long. But a lot of the others seem too likely to leak.

Well, against all odds, mine actually arrived today. It's a stainless steel cup with a screw-on top with a rubberized seal in it. As a test, I put some water in it and then flipped it upside down and shook it around. And it holds just fine. Not sure about how it holds temperatures yet.
(For the record, while the one I linked and bought is ace/aro, redbubble offers a wide variety of artistic options and you can get probably anything on a travel mug.