I had on hand:
two heating pads
I cut a hole in the top of each shoebox for my feet to enter. Then I opened each shoebox up, and placed a heating pad in each shoebox. I put my feet in to hold the heating pads in place and used small bits of duct tape to get make them conform to each foot. I cut little slots for the cords, and used some more tape to keep them from moving. I stuffed an old t-shirt loosely in each box to make the pads conform even more, but this is not necessary. I put a piece of felt on the inside heel, and a tongue, but it works without all that, so I ended up taking them out.
I like to crank 'em up on HIGH and let 'em sit for five minutes, then jam my feet inside. Oh yeah. Bring it on, Winter.