Go to you local art supply store and ask for disposable paper paletters. These are normally used by fine artists who paint in oils and watercolors. The product looks like a sketchbook and the palettes are pages of waxed paper that can be torn out and used to mix your enamels and acrylics. They are very smooth; allowing easy mixing and do not absorb the solvent. Very helpful and economical for small paint jobs such as figures or various small items.
For small painting jobs I used to employ orange juice and milk caps but you'll never keep up collecting those.