The upside to washers is lower cost and presumably better opsec on acquisition. I grid going this route with s blockmit jig but it was sized without enough tolerance for my letter stamps which had some sized too large.

The downside is the work required if ever need to destroy them. Probably easier to just dispose in geographically disbursed way.

I find metal plates are easiest to work with and their flatness also more convenient to conceal.