You can place blocks behind the shulker box when it's closing

If you are aiming at the top of a shulker box when it's closing, you can shift+click and place a block on a normally non accessible face.

Steps to reproduce :

• Place a shulker box on the ground and stand as close as possible.
• Aim at the top of the box and open it.
• Now, you need to very quickly close the inventory, and shift+right click with a block

The outcome really depend on your position relative to the box, and where you are aiming. If you do it correctly, you can successfully place a block on the 5 others faces of the box. It also works for all the different shulker box facing.
Here is the GIF of it in action: http://i.imgur.com/crOwIEt.gifv