There are two reasons for using double layers of matboard: One is strength (really a minor consideration when you think about it) and the other is the fact that I'm using up scraps of matboard in various colours so I'm putting the colours on the insides of the layers so all that shows is the white side.
The top and bottom pieces are double but the side pieces are still single layers.
The second side pieces are just dry fitted here.
The middle dividers are also double layers of matboard. They are also just dry fitted.
I'll be drilling holes for the closet rods before these pieces are glued in place.
I think that I've figured out how to install sliding closet doors. We'll see how that works out.