yes it is really heavy. For a 2x12 horizontal mesa says it's 75lbs (=35kg approx.) I haven't weighted my own, but it feels quite right. I also believe that a vertical has even more weight becasue it dimensions are bigger.
I used 18mm + 12 mm slat only on the front. This has two reasons: first is the esthetic value, c'mon it looks more brutal with a thick edge =). The second is that I needed a slat on the front to lock the baffle. The back I have only 18mm. This is because to me there was no extra value if it was 18 or 30 mm.
Oh to be exactly: it is 18 mm plywood and a 12,5 mm slat.
Hopefully this was a help.