From what I can tell in the parts pics, Jacobsen used wood blocks above and below the battery with a metal cover.
Considering this and the fact the battery would not fit all the way down on the frame, I built a steel lower platform when I first got the tractor. Yesterday, I wanted to finish this off with a cover, but I had made the lower platform too tall. I did a little investigating and found I could move the dash panel forward in the slotted holes in the frame. So I loosened the bolts holding the dash in place and slid it forward. This allowed me to rework the platform so it set the battery 3/4" above the frame just like a wood block would do. I added two 1/4" coupling nuts to the sides of the platform for some all-thread to screw into, thus holding things secure. The first 3 pics show the platform before I reworked it. I also saw an issue (first pic, center right) with a washer that could rub into the battery case, so I had added a plate between it and the battery.
I'll get a pic or 2 of the platform after rework today.
The top 'board' is a piece of the composite deck board. I used my little laminate trimmer and gouged out areas for the posts to fit into. Figured that would keep the metal cover from making contact with them. I also added 2 layers of 1/4" wood strips to allow some clearance for the vent caps on the battery.
Here is what it looks like in the tractor.
A little paint, a couple all threads, and some wing nuts will finish it off.