2x1 Piston Door (replaces top block)?
You can make it using the following steps:
Note that it is literally impossible to make the bottom of the door (i.e. the block in front of the bottom bars) be another block than a slime block or a sticky piston. The steps are, I hope, clearly shown. I didn't connect the blocks to a redstone circuit as that is supposed to be the easy part. You can do the same for the upper bars if you want them to 'disappear' from sight too.