I use cleaned pet food containers (plastic) for mixing epoxy. They are open enough to allow you to keep the epoxy in a thin layer to avoid an exothermal reaction yet deep enough to concentrate the epoxy for thorough mixing. Once dried the epoxy will release from the flexible plastic and allow the container to be reused. Mixing is accomplished with a piece of thin, stiff music wire.

Also...just an FYI, but epoxy works MUCH better if you mix equal parts hardener and resin (assuming hobby epoxy, ratio may vary with other types) and NOT equal parts of two different hardeners. Whoda thunkit....