What's the best way to store an important document in a fire/water resistant chest?

Ziploc bag, or something that seals tightly.

Warning!!! My dad had a larger (about 1 cubic ft) water proof/fire proof Sentry safe and stored old dollar bills that he had collected in it. He lived in Florida and, of course he kept it closed all the time. After a number of years, I am guessing 5 or so, we opened it and sure enough everything in the safe including his dollar bills were covered in mold. Sad.
The Fire Retardant feature actually leaches out into the safe and its contents over time.
I have the safe now and keep important papers in it. I live in a cold, dry climate and leave the door open.
You need to put the papers in clean plastic freezer bags. Also, supposedly, there is a low power stick heater that can be used. But you need to open up a hole in the safe.to run the cord. Poor design.

@LuckyGuy Thank you! That was really helpful!

@elbanditoroso That seems to be the consensus from what I’ve been reading. Thanks!

The best way is a safe deposit box at a bank.

Fully agreeing with @marinelife. A safe deposit box rents for about $25 – $40 per year. You could keep your important documents at the bank, and use the home safe for jewelry, silverware, and other, durable property.

@Love_my_doggie and @marinelife – the problem with a safe deposit box is that it is open only when the bank is open…which may not be convenient or desirable if you need papers in a hurry.