I would be careful using tweezers, for fear they would leave a mark. The resin was noticeably soft. I used cushioned kitchen tongs to fish it out of the pot.
A footnote... overnight, two of the three shells I de-warped returned to their previous state, although not nearly as bad. It looks like another round in the water, and maybe something to hold their corrected shape longer than just a few minutes held by my fingers.