I know this was sarcasm, but bank card is not appropriate because you should have one hardware key for all services produced by an independent provider.
I think your threat model is bad — the isolation of accounts offers little and most people have effectively only the security of their email provider anyway.
That said, buying several keys would be a natural and happy path solution for the paranoid. Make sure to get three for each account (same as primary) as either you need a backup or the extra key offers no extra security.