I've sparked the battery in my trunk, I'm still alive. People have been installing batteries in the trubk for decades and I've never heard of anyone exploding. They must all die, and there are never witnesses.
I put together my own battery relocation kits. I use 0 gauge ground wire from a welding supply store, I always buy 20' but always have 3 or 4' to spare because I extend the wiring for the starter solenoid and mount the solenoid in the trunk as well. This way, the hot lead from the batterey is short in length. Not wise to run the hot lead the length of the car, what if the wire housing rubs raw and shorts out on the frame somewhere? Also, the long lead goes straight to the starter from the trunk mounted solenoid, instead of to the engine compartment so it cleans up the engine compartment even more. For eyelets, I just buy a short length of copper plumbing pipe, cut as manny ends as I need. I use a vise and flatten one end, insert bare wire into the open end and give it a squeeze with channel locks to keep the cable from sliding out, heat copper with a torch and fill it with solder. Remove from vise and drill a hole into the flat end. I wrap the round end to cover any remaining exposed wire.
I buy a marine application battery box, run about $20. It comes with flimsy mounts and a weak strap. I used to get a ratchet strap and hook one end to the spare tire eyelet, and the other end to a hole I would put into the rear trunk dropoff. In recent years I've just bought the metal battery tie down strap with the 2 long J hooks. I drill 2 holes through the battery box and through the passenger side trunk foor wide enough for the battery to fit between. It comes with rubber washers, washers and wingnuts. I run the hooks from under the car, through the floor and box the the rubber washers. The rubber washers keep the hooks from falling through every time you remove the wing nuts. This all solidly mounts the battery/box in place. You don't want the battery rolling around in the trunk and shorting out. Also, if the solenoid does get trunk mounted, be careful transporting metal parts. I did this once and while driving, of all places to come on contact, a metal part engaged the starter. I run the solenoid to the outer side of the passenger trunk hinge bracket near the wheel well now.
I also have a quick disconnect on the ground wire. I like this, saves time having to tear into the box and remove battery cables.
While being transported on a flatbed cross country I left the battery in the trunk loose. Didn't think it would be a problem. Car arrived hours before I had to be on a plain to Afghanistan. A year later I get home to find this mess out. It left yellow powder, surface rusted the floor a little, dripped on the aluminum in line Aeromotive fuel filter and oxydized it, imagine the ends have fused where it threads together. Also the fuel cell breather valve tube is aluminum running through a hole in the floor, that oxydized and became clogged with yellow powder too, cut the clooged end off. I vacuumed the trunk out. There was no smell of acid but ther was still a dry residue present. I used wet paper towl to wipe it all off. Then I carefully soaked the effected areas using a bucket of water/vinegar solution, then rinsed with straight water and let it wash out. Used an air moving fan for quick dry and added a new coat of paint. Its holding up fine now. Because of this I just went ahead and painted the bare aluminum fuel cell.