I was able to to get more of the cover up, so now it is protected from the elements. I need to put up a few more boards, and then finish the joints and gaps. Then I can start on the chimney structure. I also made my own coal hook. It's a bit different than the usual shape, but it works well in my deep oven.
Assembly notes: I used Wonderboard, but did use the screws for Hardie board because I wanted the square drive. They work great and I see no reason not to use them. I predrilled holes just to make it easier.
Also, don't even think about trying full 4x8 cement board until you have picked it up. I got lucky with my dimensions, but I would highly recommend using the 5x3 cement boards and making your widest dimension 5' or less so you don't have to cut bigger pieces.
I'm hanging rock veneer on mine, so I'm going double layer. If you need to attach a second layer, or don't have a shelf to rest the cement board on, then cut a small manageable board piece to place on the bottom, and then use that as a shelf to support the larger piece on top while you screw it into place.