SKB,
I keep the plastic lid from a large coffee can on my bench all the time to mix epoxy on. After the left-over epoxy cures, I just flex the lid and it falls off. I was surprised it does not adhere better, maybe because it is not clamped. Anyway, it is always there and handy.
Mike
Mike, I do pretty much the same for mixing adhesives, body fillers, etc. It is too quick and easy to use a plastic can lid, or quickly cut a piece of plastic from a gallon milk jug, bleach jug, or drink bottle to waste time buying dedicated mixing sheets. It all ends up in the recycling bin anyway, so why add more to the waste stream? As you say, it can often be reused after popping off the cured residue. Another thing that works well is those large slick cards that all the candidates flood your mailbox with before an election. No need to try to reuse them because they send out so many. That's the only good thing I get from anti-gun Democrats.
There are many plastics like Polypropylene, Thermoplastic Olefin, etc. that epoxy and other adhesives don't stick to very well. That's why they sell chemical adhesion promoters to spray on plastic auto bumpers, etc. before using the special plastic repair adhesives. Even then, the repairs often fail, so I quit buying those very expensive adhesives and bought an old soldering gun to use just for plastic welding.