I have one just like that. It is 12 x 24 and 3 inches deep. the slots for the rulers are cut in at an angle. There are 2 posts with a dowel in the 3 inch depth one is the back support the front is where the slots are and the dowel is in the middle. It will hold your square and odd shape rulers like your 12x12 The just slide in behind the posts with the slots for your regular rulers. Hope this helps.
It also has two strips for a base that are about 1 x 7, with slots also to hold rulers so you can stand it up. I really like mine on the wall takes less room. I love my ruler holder.