Made from scratch using caspaDFG's Handbrake Mount (https://www.thingiverse.com/thing:5560775) as a model.
I had some issues with the distance of the handbrake when playing with it, so I made a new one which mounts on the same screws as the shifter plate that sits in between the wheel and the shifter.
I you want to mount it on the left side of your wheel you will need to mirror it.
The lever part is Levitron's Thing (https://www.thingiverse.com/thing:3623403)
Handbrake_Mount.stl | 42.3KB |