Mine was what I considered to be a reasonsable thickness but that's subjective as there is no max / min thickness (at least when I had the car). Was no more than 1/4" thick (think less). Material to be used? Again, subjective, right? Does one need to use aluminum? Does using another material really go against the GCR of having a bulkhead? I had bolted mine in and it could have been easily be removed and inspected if it came down to it.

I also had the spare (donut) and the associated twist jack in the trunk. I've seen people have a full size spare when it only came with the sonut.

Add back little things that were removed. Mine had rear seat belts - not much but still a few pounds here and there. Rear trim pannels. All adds up.