Give your feet a break and buy a heated foot massager during Amazon’s Big Spring Sale. The RENPHO Foot Massager with Heat is currently only $85 (normally $155), which is a massive $70 price drop.
The Beurer FM90 Foot Massager shares some ideas with those ... and while that's just an on or off setting we didn't want it to be any warmer or cooler: it worked fine through even fairly thick ...