[xmonad] Mirroring OneBig layout?

I'm finding OneBig to be a very effective layout for widescreen monitors; but my visual attention point is between two monitors; I'd like to be able to change the position of the big window. Just to top-right would be a start (I failed to understand 'Mirror' already) but potentially also to be able to nominate any corner for the big window (to accommodate the top row of monitors in a 4-screen square configuration) Can someone please give me some examples/pointers on how to achieve this? -jim, who still hasn't grokked Haskell despite loving XMonad ...