[rotorwrench]

Those Cole Hersee 2484 battery master switches are rated for 175-amps continuous or 1000-amps intermittent so they should do the job. They are also available with a key switch and a key with a leg on it so you can actually turn it.

The Aerospace industry used to have some really good ones. Cole Hersee made a large single pole/double throw toggle switch P/N MS25106-21 that was great for a switch set up for either battery power or external power with three terminals but I don't think they make them any more (they never wore out). Most aircraft use electrical relays now days.