I am so sorry to hear that!
As for the calculator examples you shared, it looks like square shapes will work very nicely for the "keycharts." I am not sure what to recommend for print's empty double low line. Maybe the visible space (⠬ dots 346) with the transcriber-defined typeform you've used for the double underline applied to it?
That would have the first example keychart and the first empty keychart looking something like this:
There may be an argument to be made for using the underline tyepform with a visible space instead of the underscores.
What do you think of all that?