Place the blocks for the display frame
Leave the front opening where the glass will move
Place sticky pistons behind the opening
Place glass blocks in front of the pistons
Run redstone dust to the pistons
Add a lever, button, or switch to power the redstone
Activate the redstone to push the glass into place
Deactivate the redstone to retract the pistons
Use slime blocks or honey blocks if you want the glass to move with a larger mechanism