IMO a fiberglass bumper is not a secure place for a battery cut-off switch. I would think that just turning it on & off over time would loosen it and damage the fiberglass.
For minimal damage to your bodywork, I would suggest a hole between the bumper & taillights for a "pushrod" leading to a securely mounted switch inside your trunk. If you back into a wall, hopefully that would actually help to shut off power from the battery.
Sorry, I don't have a pic handy of the actual switch location but I believe I bolted a bracket with it on to the ballast box. It's possible to fabricate the brackets from scratch but kits are also available. I have a rubber grommet in the tail panel where the rod goes through.
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