Mate, if I were you I'd either leave the starter solenoid in the engine bay, or use a mini starter with the solenoid on it.
Then just fuse the 0 gauge wire at the battery. No point putting the solenoid in the boot.
Keeps your wiring a lot simpler and saves you running extra power cable for no reason, plus extending the starter wire.
I'm using an automotive 200amp mega fuse from Jaycar to fuse the power cable at the battery. Similar to what some factory cars use. If you manage to blow these things starting the car, you should take it out of gear