You know those cups that come on the top of liquid cold medicine? They are great for mixing epoxy. The #5 plastic that they are made of is epoxy-phobic. Epoxy does not stick to it. Once the left over epoxy hardens, you can pop it right out.
My wife has a $200 dollar a week Nyquil habit - so I have plenty. JK
My old disposable paint brushes get the hairy end chopped off and become stirrers. I like mixing with a round stirrer. Sharp edges cause bubbles. I can spin the round thing against the side of the cup to make sure that all the epoxy gets mixed.
Sorry Glenn, but I would never mix with the brush I am going to use for coating. There is a lot of dead space in between those bristles. The only way to thoroughly mix with one is to press hard on the bristles so hard that you are then abusing your brush. Even then it is iffy.