Avanti go cup plug
thingiverse
I've got the OpenSCAD files and STL for an Avanti Go Cup plug, but I ended up washing mine down the sink since it was so tiny. I believe this part is intentionally designed to be a weak seal against spills, yet allow a pressure release if boiling water gets sealed in the cup. To install it, simply insert it into the central hole (not the drinking hole) underneath the lid flap.
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