I made it myself. Only took a modicum of sewing - thank goodness or else it would not have happened. :)
Step 1: Take one clean, holey sock and cut off the part with the hole. (In my case, part of the foot near the toe.)
Step 2: Sew up the part that was cut and turn sock inside out so the nice seam faces out.
Step 3: Fill with rice. I used broken rice (because we still have a most of a big bag that we bought by accident and it's only good for congee, which I only make for Max and Marvin because I don't like to eat it).
Step 4: Sew up the other side of the sock. Make certain that it is securely sealed. I sewed two rows to make sure.
Step 5: Place rice pad in the microwave for 1 minute. Check to see if it is hot enough. If not, nuke for another minute.
Step 6: Knit a machine washable removable cover to fit over the pad. This hasn't been done yet but I can tell you that I used the hot pad several times last night on my neck and it feels wonderful.