There are several published examples of ways to glue or lay on top of a normal keyboard to get a quick, cheap and dirty way to experiment with Janko type keyboards.
Plastic molded/ d3 printed appear to too expensive in small runs to encourage many people to try a chromatic keyboard. With a part list worked out cut wood (or cut plastic?) pieces might offer an affordable way to make a kit hobbyists could use to modify a keyboard in an hour or two for a low enough price to get wider exposure. A good goal would be less than $50 to do 4 octaves. It is desirable to offer different key shapes and markings both visual and tactile.
Problems to overcome:
The tall structure over the keys makes a big lever to twist and break keys if they are bumped from the sides.
Noise of new parts moving.
Weight changing the action of the keys.
Irregular spacing of the underlying keys necessitating off centering of the key tops and possible instability.
Keyboards do not have standardized key sizes. The bigger the original keyboard, the better.
For removable adapters, stability of something hanging rather far over the edge of the keys. I.e. when the bottom row of keys is pressed the whole thing should not jump up on the back. Also a way for the unit to sit in the correct spot and not move around.
Here are links to several
This one glues directly to the keys where possible
This one has longer fingers glued to all the keys
A removable adapter to use any keyboard as a Janko.
Pre-built adapter $135 per octave