My problem with the thumb keys on the CyKey was that there where three of them and they needed to be pressed in six different combinations, three single and three pairs and this demanded difficult contortions of the thumb.

To overcome this I built a keypad in which the thumb could move in six different directions:

This was better but the mechanism was a bit complicated, so I built a second keyboard where the number key could be operated by both the thumb and the first finger:

Better but still requires Thumb to press two keys at once to cancel upper case.