Like some others, I think that computer keys appear to be the screen-used item, but we could be wrong. The only problem is, most of today's computer keyboards have 'slanted' keys, so you'd have to sand or file down one edge to make them flat.
I imagine someone will eventually stumble onto the actual screen-used buttons, much like they did with the gauntlet calculator keypad.
Maybe an adding machine, or something other than a computer keyboard? This was back in the early 80's, so...
And that plumber's putty is da bomb, I've used it before. Anyone tried filling the computer keys with the putty, then sinking a threaded bolt into it to dry? It seems like that would work.