A Kill Switch is REQUIRED.
It is considered safety equipment, and therefore exempt from the budget.
The Kill Switch should be easily recognizable as such, so that a Track Marshal or any racer can easily disable a runaway car.
Many cars use the HELLA 2843 Series 100A Battery Master Switch
There are cheaper ones on eBay, usually listed with names like 12V/24V Car Truck Boat Battery Isolator Disconnect Cut Off Power Kill Switch.
Even Harbor Freight has one for less than $10 - Battery Cut Off Switch.
(Milwaukee Makerspace had a big pile of e-stop switches so we've used those on many of our older cars. Typically the e-stop switches aren't rated for high-current, so we use it to control a relay that can handle the high-current.)
Phantom Power Racing uses the well specified ED250 style kill switch with magnetic blow-outs: being able to break 1000Amps guarantees you won't have an arc that melts the switch shut. The ED125 is probably less over-kill for the application. They can be found on aliexpress, ebay, etc. for $21-$23 plus shipping from China. I've been warned not to mount them with the button pointing up as there is a risk of them shutting off if you hit a bump.
Though not used as much since 2014, some cars also have a Dead Man Switch. Typically this is a small rope that goes around a drivers wrist with a wire loop and an Anderson connector. If the driver becomes separated from the vehicle (or yanks their arm too far from the steering device) the connector disconnects killing power to the car.
This is a schematic of the wiring for a 36v battery and a 12v main contactor. (Notice that the relay coil is only getting 12v from the 36v pack.)
Make sure your heavy contactor can handle the current and is designed for continuous use. (Note: starter relays will burn up if left turned on!)
Here's a 12v continuous duty solenoid in an insulated steel case that can handle 80 amps. The 2 pin connector can be anything, but Milwaukee Makerspace has had good luck with these Anderson powerpoles on short wires.