This was in regards to the concrete board... did he give any reasons for that? Is it supposed to absorb it better or have a better bond or something? Knowing what you know now (that you had to use double the amount) would you make the same decision? Do you think the concrete board contributed to the difficulty covering it?
When I buy (also in Naperville, most likely) I am going to copy your build almost exactly, though with the added twist that I'd like to be able to make mine removable. I'd be interested to get your input on how you'd have modified your design if that was a consideration. I'm kind of thinking that the extra layer provided by the concrete board might make it easy to build everything else in plywood as a multiple-piece shell and a move would only require breaking down the board on the inside and re-sealing all the seams when setting it back up. No clue how realistic that would be though.