I think what John means is that if he use tee hinges and the art work is landscape, when he comes to store them portrait, because there is not enough room to store them landscape, the weight of the artboard will cause it to move in the frame. I had the same thing happen to me many years ago, with almost a metre sq Limited Edition print was mounted and framed, somebody turned the frame 90degrees (on the Friday) when I came in on Monday, the print had moved
John what you need to do is make a 'sink mount', after you have cut your window mount, place your artwork where you want it on a piece of mountcard ( which we will call an undermount)the same size as the window mount (external size), now with offcuts of mountcard or Foamcore, glue these (which we will call them filler strips) around your artwork to the undermount, until they are the same thickness as the artwork, now they are touching it and holding it in place, now stick your window mount on top, you can use glue or double sided tape to stick the window mount to the filler strips. Put in your frame, and you can store however you like.
I have use this method on many types of artwork which are heavy such as screen prints on aluminum, wood and glass.
Hope this has helped John