Thanks, Chirpy, that's exactly the size that I was hoping that I could make it. Yesterday I bought two of those $10 black-plastic 2x3-foot concrete mix boxes to use as poop catchers under my roosts. I'll buy another to use as a dust box. BTW They're 24" x 36" x 9" deep. I'm going to put a frame of 2x4's around it to help keep the dirt, pete moss, ashes, and diatomaceous earth from being knoched out.
I bought one of those plastic storage boxes that will fit under a bed and filled it with sand. The birds loved it. Originally I had it out in the run. I would put the lid on it when rain was forcasted. I forgot a couple of times so I put some small holes in the bottom so the sand wouldn't sit in water.
I have mine sitting under my garage's eaves in the run that has almost a foot of sand for a run. It's in a spot where it gets sunlight all day. It will have to be a very heavy rain with wind blowing before the dirt and other stuff gets wet. When it does get wet, it should dry out quickly. The sand in that area has not gotten wet yet though.