[xmonad] Mirroring OneBig layout?

[Jim Cheetham]

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 ...