You just decide what size rubs you want draw it out, turn it into a drawing with measurements and then work out the pieces of wood required to do it. Or buy ikea furniture and modify it,2 billy bookcases fastened together give the depth required for rubs.
Here you go, heres one of mine. Its chucking it down with rain at the moment but when it stops i will go to the garage and get my tape measure and put up the measurements. This one is in 3 bits, 2 x 2 layers of 33ltr tubs, and the top layer with tubs for hatchlings. Makes it easier to move if you ever need to,