I finally got the deep cycle battery properly secured in the trunk. These photos should be mostly self explanatory but I'm sure I'll find a few words to add...
Remember this battery pack weighs only 16 lb. The aluminum framing is certainly up to the job of holding it in place.
Aluminum 1" x 1" x 1/16" square tubing in place
The three screws into the trunk floor are #8 sheet metal screws. The six others are 1/4-20 bolts with nuts + lock washers. You also see a 1/4-20 threaded rod with nylon-insert lock nut on the back side.
Battery secured in the framing
J-hook rods are pretty standard for battery holddowns. However with cramped access and the rods going sideways into the 1x1's it was much easier to set up a locking nut on the back side of a threaded rod.
The pack is 7 3/4" long, more than the width of than a typical automotive battery. So I needed the adjustable black plastic bracket, at its maximum length.
Trunk flooring back in place, laid up against side of battery
