I've got a design for you. It's probably overcomplicated, but whatever.
Here's some of the things it does:
-zombies slowly break down each piece (one of the four slabs) over time
-change the time it takes by changing the length of the orange clock
-you can press a button to repair the barrier. I have it set to one slab, you can change that if you want.
-you can 'overheal' the door. So it takes more than 4 clock revolutions to open (up to 15)
-'overheal' is technically a bug, but I couldn't be bothered to fix it, and it might be useful
Anyway, I included a MCEdit schematic
here.