After getting tired of spilling paint and pounding the lid back on the can I recalled a pour/mixing lid from when I ran a paint shop in high school.  I did a quick Amazon search and found plenty of options, even some claiming to be approved for use with solvent paint.  Cost on Amazon was in the $20-ish dollar mark for a gallon lid.  I went to Ben’s Paint Supply and for $11.75 I was out the door with a known quality lid.  They are designed to be used with a mixing system (the likes of which I have never seen) but can simply be chucked up in your favorite drill and mixed without effort.  The spring loaded pour spout is wonderful as well so you can easily pour an exacting quantity without dripping it down the side of the can or over pouring.  They come in quart size lids as well.

Here is the lid in its designed use.