Not as elegant as what is being suggested, but you could always get this. Note it has a 20-25A limit.
https://ebikes.ca/shop/electric-bicycle-parts/wiring/keyswitch.html
Do you have an ebrake lever that you want to serve as a cut-off? That is not a feature on the Grinfineon controllers, unfortunately. If you don't have a lever, then no worries. Your throttle does not go down to the regen voltage range when you release it.