It's been done before (I've seen it years ago), but maybe Ben could make something more refined and add some ideas of his own.
But, basically, you would have a stand or box in which you could attach your ordinary graphics tablet to, that would have an LCD screen upcycled from a broken laptop with a sheet of glass or polycarbonate on top of that. The stand would make it easy to remove the tablet if needed, and would also allow for different angled positions for the artist to use it more comfortably.
The glass/polycarbonate sheet should be somewhat thin in order to minimize the parallax effect.
I'm thinking of something built out of wood (lasered, maybe?), sintra or acrylic rather than 3D printed because the stand would have to "stand" some weight when the artist rests his arms on top of the tablet.
As a bonus, it could have a handle to be carried around.