What I do is treat it like the dodge feat - The invisible disc interposes itself between the wizard and one particular foe, so it gives the +7 cover bonus against that one foe (no matter how much that foe moves). The wizard can reassign it to a different foe as a free action on his turn.
Simply and has worked well so far.
This method means that you don't have to worry about facing (which the book version foolishly re-introduces), and it is a useful option if the wizard really wants protection against the big nasty rather than his underlings.
Cheers