I like to pre-load my colors using this device before heading out to plein air paint. Then I clip the small palette to my mixing palette as an extension. When I'm done I put the bottle in the freezer for re-use later on. It should keep for several days, if not weeks, depending on your oil brand of choice.
I have to say that you can buy a similar product called a Garage Palette for about $22 if you don't want to go to the trouble of making your own. The Garage Palette website recommends using clove oil to help slow down oxidation so I suppose that a couple of drops on a cotton ball placed under the palette would work just as well. Other oils might also work but I haven't tested any.
What you need:
Thermos Water bottle (about $8)
Back saw (or saw of your choice)
Piece of cardboard or mat board
Wooden palette (preferably used)
First, cut a piece of cardboard or mat to the size you need. This will be your guide for cutting the finished piece.
Don't make it too snug since you may have some paint build up over time that will require you to sand it down.
Cut your wooden palette to size. I used a back saw which worked well.
Super easy and keeps your paint where it belongs.