I’ve had the same problem with Polar Pure, and I think I’ve come up with a solution. I put the iodine crystals in a small glass vial, wrapped electrical tape around both ends, and slipped the vial back into the Polar Pure bottle. The vial is long enough not to get stuck under the plastic barrier and the Teflon seal and tape prevent leakage and rattling. To test this setup, I placed the bottle and a paper towel into a Ziploc bag and after a month, there’s no discoloration of the paper towel. As for a cap, I’d try a vitamin bottle.