My uncle Leo told me this great epoxy tip: "if you have an epoxy leftover, put it in the freezer". I do this mostly with epoxy mixed with some silica powder prepared for glueing. It works, also with unthickened epoxy! He also cools the epoxy in a box with ice, to prevent curing of the epoxy and herewith lengthen the working time.You only have to be alert not to get ice water in the epoxy.
I also put my epoxy rollers in the freezer, so I don't have to throw them away after I've used them once. Saves a lot of material or cleaning.