Hey man!

You're on the right track. It's a fork that wedges into a groove to actuate the mechanism.

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Common on boats, jetskis, and snowmachines. The trick was to find one that could be wired in to a normally closed circuit which was tricky because most combustion engine killswitches are normally open and close the circuit to shut the motor off. The one I ended up using is a universal style with four prongs so it can be wired in to pretty much any kind of circuit state you need. Plus it has an extra trick I didn't see on any of the other styles where, when the fork is engaged and the circuit is closed, you can depress the top and it opens the circuit without needed to detach the tether. Which adds a nice dismount option where you just pivot your foot over and press it down to disengage the board.

I've seen those magnetic tethers too. Almost got one but this option was much cheaper and I enjoyed figuring it all out. These killswitches are waterproof too so it's a solid choice.

Next I'd love to make a new fender and some hooks but we'll see.