With the transmission fixed, my spirits rose again and I spent more time in the shop, finishing the battery bay. No rocket science but an important step. Also the brackets mentioned in the video, which will mount the rear part of the bay to the car, are already done and ready to be mounted. After checking the distances of the batteries again, I found out that it should work within the tunnel and on the left side. But the ones on the right side of the car, exactly where I'm working on in the video, are most likely a bit too close and I have to re-do them - sacrifying a bit of the safety space towards the rear strut. With some insulating rubber or silicone it should still be allright though.
To prevent the batteries from moving up in the construction, I first thought of velcro straps. But I think I'll be placing a plastic sheet on top of the batteries and then fill the space above with expanding construction foam. This should sufficiently hold the batteries in place vertically. The major forces will be picked up by the aluminum construction.