I have been using epoxy for over 25 years on various boat projects. Thicker plastic, say the thickness of a ziploc bag, works well and epoxy doesn't stick to it. There will not be any residue left on the epoxy surface from plastic as there can be from wax paper. On some occasions I have filled a surface with thickened epoxy and taped plastic over it to keep the shape and when it is removed there is virtually no sanding required.
I agree, anything plastic works, as long as it is strong enough to be removed without tearing. Saran wrap isn't strong enough. The tools to work epoxy, from containers to mix it in to putty knife type products are all plastic and very re-usable.