I do also think you need a better form of battery tie down. What I would do is cut a plate of 1/8" steel to go under the current location of the battery, and weld it to the floor of the car under the fiberglass. The get a crossbar type batter tiedown. That will come with two long bolts, whch could be secured to the plate.Another alternative is to get a dedicated battery box, and bolt that to the floor underneath the fiberglass.

The main concern of a good battery tie down is in the event of an accident. You certainly don't want a heavy object flying around, or leaking acid (not with an AGM). But even though this is in a seperate compartment from the interior, in an accident like a rollover, that battery could end up hitting you in the head the way it is secured now. That slide out tray is hardly adequate protection. The other issue, is that a battery that can move can end up arcing on something, and igniting a fire in an accident.

Over the years, I've run tech inspections for many years at BMW CCA and SCCA driving events. That set up would fail.

__________________
2014 BMW 328i Xdrive
2011 BMW 335i M-Sport
2008 BMW X5 4.8i Sport
2000 BMW 528i 5sp