Here's try number 2.
I had to cut out a couple of the supports in the back because they blocked the plugs. DOH. We've adjusted the supports for the 3rd so there will be just 2 vertical supports in the back maybe an inch inside the corners. This is to allow the primary battery plugs.
I added 3 layers of 3M automotive tape to the left side and 2 layers to the other two sides to compensate for the contours of the deck lid. I don't think I'll need to do this to the other cage that will go on the right side of the trunk.
Here it is attached to the deck lid with and without the battery.
I'm leaving the battery in the cage overnight and will see if it's still attached in the morning. I don't have any reason to believe that it won't be. It's got a lot of contact area. I should have the next run sometime tomorrow and I'm going to try to install it equidistant to the trunk light on the right.
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2022 M3 Comp BGM/Fiona Red, Executive, Drivers Assistant Pro, Parking Assist, 826M black wheels
(SOLD) 2018 M2 BSM LCI, DCT, Executive, Drivers Assist, CarPlay