Something using a sequence of normal and sticky pistons to first push the blocks towards the center, pull them in and then pull them away for storage would be my first idea. Not certain if the wiring required would be feasibly though since the pistons would need to be very close to each other. Or just be boring and have two rows of BDs facing each other with 2 spaces between with glass and the wall block in them, but that's the cheap way out and not entirely the same thing :)