For the speed control of my Maxun One solar bike, I use an up-down thumb-throttle switch. Normally, such a throttle is mounted between grip and the brake lever which increases the distance between them, see for instance the Bionx throttle or the Shimano switch unit SW EM800 L. That's not handy. That's why I made a throttle that is mounted on top of the brake lever clamp so that no extra space is needed on the handlebar.

Sensitive tactile switches

Since you have to press the buttons very often, very sensitive tactile switches are used so that you only have to press very lightly: the Panasonic EVQ-P6DB35 tactile switches are also watertight (IP68)

 These are the parts:

 This is the throttle with the PCBs glued on it:

 Glue it to the brake lever with 3M VHB tape:

 Side view:

 The throttle internal wiring:

Contact me for the design files.