I broke the cap of our Crofton Thermo (looking pretty much like this https://ae01.alicdn.com/kf/Hdcc7af1515d9493784225b77a21aa6a58/Water-bottle-thermos-Double-wall-Stainles-Steel-Water-bottle-Thermos-bottle-keep-Hot-and-Cold-Insulated.jpg).

So I designed a replacement and printed it with PETG.

Works pretty well so far. Remember to use the O-ring off the original cap, though!