When I put my battery in the trunk a couple months ago I used a Ford (single small post) type solenoid along with a Painless alternator disconnect on a panel at the battery.

My starter cable runs from the solenoid directly to the starter so it's only hot when cranking. The factory solenoid was left on the fire wall with a hot wire from the disconnect switch in the back feeding power to the post that the + cable was on. Everything except the alternator and starter get power from there. There is a wire from the "start" terminal of the factory solenoid to the single post on the Ford relay in the trunk. My car still has the three post neutral safety switch and wiring, it starts great set up this way.