Ok, I am trying this out in SP, to see if I can get it to work, but the wither keeps escaping out (obv the front was sealed in, I just removed it to show the structure)...
the design you recommended above didn't work either.
Interesting...I did it just before I posted originally, and had no problems. I have no idea how he could be escaping. Maybe try adding in the corners, or doubling up on the walls?
Is this SSP or SMP ( especially if it's a server online)? I wonder if some sort of server latency or something is allowing it out.