I agree that the AI you envision would be dangerously likely to escape a "competent" box too; and in any case, even if you manage to keep the AI in the box, attempts to actually use any advice it gives are extremely dangerous.
That said, I think your "half an inch" is off by multiple orders of magnitude.
Update 2013-09-05.
I have since played two more AI box experiments after this one, winning both.
Update 2013-12-30:
I have lost two more AI box experiments, and won two more. Current Record is 3 Wins, 3 Losses.