I'd go for something like the cryptosteel cassettes. I bought a kit that uses the washers but had nightmares that I went to recover my keys and the washers spilled on the floor.

You should test/validate your backup strategy by getting it all put together, tamper sealed, locked etc., then opening it and trying to recover your keys. You should also put any close family members through this process too. If you have to disassemble the thing you are always taking a chance that you mess up putting it back together. The cassette type is nice because the letters display without needing to disassemble it, no ambiguity, easy to build and validate.