For my Jeep CJ7, I took a military ammo can, bolted it through the bottom to the rear fenderwell just behind the roll bar mount. I put silicone caulking around the bolt heads and put a hasp and keeper on the ammo box. It creates a tough, waterproof and secure storage area for gear. If you need more room, put one on each side.

This is also a secure way to mount rear speakers. Just cut the speaker hole in the side of the ammo can, mount the speaker on the can and lock the lid down. The best thing about it is that ammo cans are cheap and waterproof. Hope this helps.
George