I use a reusable silicone cup to mix epoxy, pour, and then I just let the unused epoxy harden at the bottom of the cup. Then gently pushing from the bottom, it comes right off. I usually have to do some extra cleaning to make sure there are no other residue, but it works pretty well.
The beak and the ability to squeeze it when pouring is also useful, much easier to control than a large rigid cup (for clumsy me)