I did that, and it came out to $19 w/o shipping, $32.06 with (slow) shipping to the US, for qty 5. That breaks down to their usual $4 setup plus an extra $15 charge for the board size, but overall not bad.
To keep this keyboard-related: this is also roughly the size needed for the bottom PCB needed for a full-custom keyboard layout (a little wider, not as tall), so feasible for that as well. Dropping the thickness down to 0.4mm for that purpose doesn’t increase the cost at all.
So very cool. Might be something of interest for the wider community to replace the top plate, potentially. For a future thing, after you’ve got the keyboard mod working the way you want for yourself 
This looks brilliant. I would absolutely love to have one, given the fact that I’m an avid VIM and Sway user, and would purchase the parts needed as soon as they’re available!
That prototype looks exciting. What’s the next step here? Can we help?
Is this also possible on the Laptop 13? I’d love to have the Thinkpad keyboard back 