"If I want to switch it with a sensibly sized switch,presumably I would need a relay,as 90A @12V is a pretty heavy load."
I use a 50A latching relay together with a blade fuse in my Crash Tender to switch the ESC power. Using a latching relay means the relay doesn't continually draw power, only a short pulse to switch it on, and another to switch it off. The relay has two coils, one for on and one for off. These are activated using a small centre off spring loaded toggle switch mounted under the sliding hatch on the forward cabin roof. Push one way for on, and the other for off. The second switch is for the separate receiver/auxillaries battery.