I've been putting my left over oil paint on a piece of glass that came out of a mini sandblasting machine at a dental design factory. This glass fits into the lid of a shoe box so I've taken two lids - one is larger than the other so it becomes the lid to the other.
I've used both of these lids for palettes while painting with acrylic paint, so they are well coated with a skin of paint. This makes them water proof for storing the oil paints in the freezer.
The paints I've squeezed out of the tube don't dry up as fast when they are stored in the freezer.
Just today I realized that this lid set-up will be perfect for me to carry around with me when I go out on my paint excursions. I'll need to clean off the glass and put paint around the edge so I have place in the center to mix colors.
My next step in getting ready to go to the 'field' to paint is to come up with a way to carry my medium and brush cleaner without making a mess.