If you are collecting it in any quantity for sale, they make a propolis mat. You place it on top of your hive in place of your inner cover, and set a solid bottom board on top of it. The bees seal it up to eliminate draft. Once it's full, simply remove it, place in a freezer overnight, and then remove it, flex it, and all the
clean propolis just falls off. Then put the mat on another hive.
Most of us have a hive or two that seems to create proplis like mad, and these would be great candidates.