At one time I wanted to build a completely separate structure, like a freestanding small closet or large dog house, to store gas and diesel fuel cans in. Get it completely out of the garage and shop. R panels, hardy plank siding, put a plastic catch basin in the bottom or just fill something with kitty litter. I was thinking pallet size, so I could pick it up with a tractor or fork lift and move it, so a short, squatty sized dog house that opened from the top would make more sense.
So if it did catch fire, it was enclosed but out away from everything. A fuel dump.