best way to store encryption keys...

If I wanted to store encryption key (900 characters), which field is best for it?

Text field seems too short.

Secure note is a possible option, but I wish to have note "hidden" or blurred until I click on it to reveal.

Or just have it attached as a file??

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1Password Version: 8.4.1
Extension Version: 2.1.4
OS Version: win10 x64