I didn't vote, because I don't have a MEC press. I do have several RCBS UniFlows and a Lee Load-All, however. And I don't store powder in any of them. First, I don't want to etch the plastic on the hoppers. Second, if the seal on the original bottle is intact, (to re-use) the powder will not degrade quickly, if at all. The hopper is not air tight. Third, I was always taught to store powder in the original container to prevent mix-ups and the like. Only times I have left powder in a hopper is overnight once when I didn't have time to finish a current batch.