Momentary: Starter solenoid (they usually have a limited duty cycle)
Sustained contact: Industrial power contactor (Relay)
Battery Selector switch as used on Boats -(usually four position Battery #1, Battery #2, Both, Off)
Check an auto parts store for a battery switch - usually designed to mount on the battery terminal to break contact to prevent loads and leakage from running down the battery while in storage, or to prevent vehicle theft.
A Big Old knife switch - scarcer than hen's teeth, but relatively easy to make.
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