I also found that the groove filled with paint after pouring out of the
can and it was a PITA to get it cleaned out.  As a result, the paint went
bad pretty quickly from water in the atmosphere.  I bet one could pick up
a cheap plastic dessicator from a used lab equipment store online.  You
could use the manifold vacuum from the engine to clear out ambient air
then shut it tight!

I also like the sheet metal screw trick.  How about making a slightly
larger hole and plugging it with a rubber stopper.

jake