A few years back we created a Shin-ei Uni-Vibe Expression Pedal as a replacement for the original controller which can be hard to find. The Uni-Vibe requires a controller to adjust the rate and for bypass so if the controller goes missing or is damaged then the Uni-Vibe is useless. The one picture above offers the same functionality but in a smaller 2.5″ x 4.5″ package that’s designed to be activated by hand rather than with your foot.
Edited to add: check out Part 3 for another variation on this device.