I did this for a while. While tab-per-tab, iodine is a better buy than chlorine (maybe), since I'm rotating them every six months rather than every two years in the emergency gear and then a year in camping and hunting stocks... hmmm... I carry clorine dioxide.

And the items you are looking for are called septas. I've got some in storage at my folks. The seal is better than nothing, but imperfect even with teflon tape on the threads.