Our chickens are going to be in a permanent run so I want to make a good sized dustbath for them that will be in the run and under cover so it doesn't get wet every time it rains. But what size is a good size to make it? There will be 11 chickens (whoops, I blame chicken math) ranging from a standard polish to a buff orpington in size. I know they won't all use the dust bath at the same time, so maybe big enough for 3 at a time?? What do you recommend? FYI I'm hoping to construct the outer frame and walls from scrap 2x4s and fence boards.