When I removed the gas tank from the car several months back, I discovered that the gas tank was used as part of the trunk enclosure, and a big hole would have to be sealed.
I cut a panel out of 18 gauge (1 mm) steel. The back of the panel had to be shaped with some persuasion from a hammer. The panel was then welded into place. I used a bright light from underneath to check for any pinhole openings.
Some primer and paint finishes the job. The underside is finished in primer and a high-build undercoat spray. This is the first sheet metal work I’ve ever done, and I’m glad it will not be very visible once the batteries are mounted in this space.