If I only need to use this for an hour or two, can I seal it in a ziploc and re-use it later?
These warmers cannot be reused.
Grabber actually sells a new version with a little ziploc bag for this purpose. Works only so-so.||The heat is produced through oxidation of the contents so when you seal them up and exclude oxygen you can slow down and stop the process. When you reopen the bag, the process continues. Unfortunately ziploc bags are not necessarily air tight so sometimes a second use will not work well, especially if they have been sitting for a while because oxygen slowly leaked in.