I recommend the Charcoal Yellow Clay Hot Pack.

Since I do physically demanding work, I never have joints that don't hurt from my waist to my shoulders ㅜㅜ

So I used to heat a hot pack in the microwave, but it was inconvenient because I had to reheat it as it cooled down. On a friend's recommendation, I started using the "Charcoal Hot and Warm Clay Pack," and I love it. You can wash the outer cover when washing, it's electric so it's not bothersome, and it has a 1-hour automatic setting. I use it twice a day, and when I can't go to the orthopedic or Korean medicine clinic, it's great for joint pain.