Click button for the Primus Commuter Mug.

It will fit both the 0.3L and 0.4L mug. The picture shows the 0.3L mug.
The STL-file does not include the "C" on top which shows in the pictures, it is just flat with a chamfered edge.

It might be best to print this in ABS, as the PLA is more sensitive to heat, but I've had no problems with the PLA-version.