Not the most constructive response, but are you expecting to use two filters PLUS a circular polarizer and have no vingetting at an extreme wide angle? I would assume that as long as the filters are covering the front element + some room you'd be fine....but that seems to be asking a lot out of a system. IMHO
You might want to check out http://www.lockcircle.com/primecircle_xt_lenses/
and the matching http://www.lockcircle.com/matbox/
I know it is "cine" but it seems to be an effective system for a customizable hood, and filters which seem to be the best solution I could think of.