Thanks to Machka's suggestion the MEC nylon booties have been doing the trick for me. I had to go a size larger than the recommendations to accomodate my Shimano mt. bike shoes. I also wanted room to pull a sock on over my shoes and still fit inside the overboots. I eventually found something even better than a pair of old wool socks. I pulled the thinsulate liners out of an old pair of pac boots, cut out the soles to match the open sole pattern on the MECs, slit them up the back and pull those on over the shoes before I don the overboots. That combo has been pretty bombproof even in the single digits F.