Ok, I’ve had an even better idea! I’m still thinking about it but basically it consists of a couple of very large copper contacts and a series of neodymium magnets. You’d have the contacts arranged such that when you put a second set of magnets near the first set the contacts close.
In this way the relay-like assembly would be inside the battery box and you’d have a set of magnets in a small housing that you would simply place on the correct spot on the outside of the battery box. Call it a puck.
So no actual contact between the puck and the relay except through the plastic, thus keeping the battery box waterproof. You’d have a puck positioning ring glued to the outside.
The puck would be on a tether. So while the puck is in place contact is made and the magnetic attraction holds the puck in place and keeps the relay turned on. But as soon as you fall off you yank it away and the relay disconnects.
There’s only 2 things I can think of that could go wrong. One is the puck smacking you upside the head when you fall and the other is you’d need some anti-spark at the midpoint of relay contact so that every time you re-armed it it wouldn’t spark and destroy the contact points. Or you could have a bleeder resistor and a hard cutoff. That way the mechanism would be a lot simpler.
I think I just found all the parts I’d need to do this. Behold, the starter solenoid repair kit.
All I need now is some magnets and some quality 3D printer time.