Relay looks like this:
Buy a
kit like this from Summit. Then you get a switch, a relay, some wire, a breaker & a relay. Get another relay locally, and you will be well on your way.
You may be able to cobble it together cheaper, but not much.
Wiring Diagram
*purple wire needs to be fused at source
*I might use breakers instead of links for the main power - not fuses though.