For my hold-down, I bought a length of flat “strap” metal, I guess you’d call it. It’s about 1-1.25" wide, and maybe a 16th thick. Has about a 3/8 hole spaced every 2 inches or so. In the metal rack at Home Depot.
Definitely bendable but still somewhat stiff.
Anyway, on the left side of the battery I drilled a hole in the raised vertical lip of the trunk floor and attached with nut/bolt, and then I ran the strap across the battery, down the right side, and drilled a hole down at the bottom and fastened with nut/bolt. I think given your situation, that may be easier to deal with than the more elegant threaded rod.