Open top 4 x 6 cage made from 16" high closet shelving and coroplast. Note: the baby in the corner is NOT trapped. I made him a nook so he could get away from the big piggies. He easily "pops" thru the grids at will. As he grew I made a false front for one of the wooden houses that only he could fit thru so he could have space when he wants it.