They make some magnetic switches with no moving parts for car alarms that might work great for your application. Under $10 on eBay.Just an FYI, I used one of those pin switches on mine I made last year and only after a month or two there was either enough corrosion or arc pitting that the switch quit working and you have to wiggle it to get it to work.
But I am up in the 'spray rust and corrosion enhancing chemicals on the road to prevent ice when the road is totally dry' part of the country so I am sure that didn't help at all
Other that that Nice kit!