High temperature bath chemicals
Different types of solder can give a very high operating temperature, are cheap and offer excellent conduction and heat contact when molten. We have used lead and cadmium free solder to 500 degrees C with no problem. Your issue may be the melting point (ours was 230C), but the right mix may bring this down for you. If you have the time and inclination, you could modify a commercial mix yourself. Otherwise,  sand would get my vote.