Just to point out be careful of putting anything that applies direct heat to your feet, I don’t know if you have any loss of sensation or feeling in your feet but if this is the case you may not feel that the heat is too hot and this could cause damage to your feet that you may not notice and create a lot more problems.
My hubby always has cold feet (not diabetic) he always puts his socks on the radiator for a few minutes before putting them on, my friend who does a lot of fell walking always swears by 2 pair of socks to keep his feet warm, one thin pair a bit like silk then a thicker pair on top (I think they may be specifically for hiking/climbing so maybe check out hiking socks) apparently it creates a layer of heat between the two layers, don’t know how true that is