You might try layers on the feet. See if you can find some thin (silk?) socks, and wear the wool socks over them

What are your shoes like? If they're the typical summer bike shoe with lots of mesh, you can go with a plastic bag overlap (is that what you meant by ziplock?) or go to a winter cycling shoe. If you're using platform pedals, big heavy work boots will help.

Also, make sure you don't have gaps on your legs. Half an inch of skin around warmers doesn't sound like much, but it ends up restricting blood flow to the feet.