Have you thought about using polymerclay to make palettes in Altoids tins to hold watercolor paint? It's easy to do and there are tons of instructions online to teach you how. I bought white polymer clay, stuck it in the bottom of the tin and pressed a pencil end into it to make wells to fill with paint. You cook it in the oven for 30 minutes on 275. Then I painted it with some acrylic glaze and let it dry. Then I filled it with paint.
The palette worked well! I am now thinking of more ideas for containers to hold portable palette. Do you have any suggestions?