Rack for water bath
A canning rack is required if you use a glass canning jar or Pyrex glass measure to melt the ingredients in the water. The rack allows water to circulate around the bottom of the glass so it will heat evenly and prevent breakage. Glass can break if hot spots develop.
The picture on the left shows a regular canning rack inside a pot. The middle picture is a homemade rack made from canning rings and twist ties. The right picture is a round cake cooling rack.
Another suggestion is to punch holes in the bottom of an aluminum pie tin and place it upside down in the pot.