I am not a bike mechanic. I found a discussion of a similar issue at
https://forum.cyclinguk.org/viewtopic.php?t=156474 where people recommended 3D printing something, or fitting bar-end shifters to use for the control. If there is a Maker-type lab or an affiliate of
https://moveunitedsport.org/ near you, they may be able to offer suggestions, or to help with design and fabrication.