Those stair forms are going to make your display quite heavy so if you plan on making this mobile, I would look at something a little lighter. Depending on how many phones you wish to display, 1/2 to 3/4 inch plywood could make you some nice shelves. Hold it all together with some pocket holes to hide the fasteners and do your own veneer job on the exposed plywood sides. I was also thinking along the same lines as Ed with hiding your cords, that way you can coil the excess in large loops so as not to damage them.