When I built a CNC router, my hardware e-stop button (I made sure to get the largest red A-OO-GAH slam button that McMaster-Carr had) was simply a normally-closed switch, and when punched, it would briefly open and cut the power just long enough to make the system reboot and kick it out of whatever program was causing trouble.
Should be easy to wire in-line with your main power line. I would suggest a separate weather-proof box (Lowes, Home Depot, etc.) to house the switch and power connections, which can just be mounted to the side of the bed.
Or, you can do what I did on my latest ‘bot, which was to plug the power line into an internet-connected light switch (15A load limit was more than enough). Then I could cut the power from anywhere. Also very handy when things went haywire and it needed a reboot. No more pulling plugs; just open the app, turn off the light switch, and turn it back on. Especially handy since I’ve done some ‘bot installs around the community for schools and non-profits; the 10 seconds it takes me to reboot from my house is better than the half-hour spent driving to the machine just to unplug and replug it. Plus, you can control it with Alexa or Google Home… Or the Apple one, if you’re a rich masochist, lol.