Battery relocated to trunk, inertia switch?

A battery cutoff is not normally a solenoid, because every time you parked the car, and did NOT kill the battery, the solenoid would be drawing current. Unless you are willing to deal with that, I don't know how you'd integrate a low current rollover switch with some sort of cutoff

One thing I've considered is to use an "easy pull" cable such as a bicycle shifter. You can buy that cable and housing in bulk. Run the cable from the disconnect to say, between the front seats with an unobtrusive handle down "in" the carpet. You can just reach down and yank it and it will be "off"