Sorry, might be too late, but I use a set of Serratus overboots that I got from mec.ca, they're nice and can fit over MTB shoes (I use them on my Diadora Jalapeno IIs with Time ATAC cleats). They are waterproof and windproof (except at the cleat opening or underside) but they aren't like real form fitting like some overboots so they can be used with bigger shoes. With a velco strap it is somewhat flexible in that regard.

Jay