Remove all attachment e.g. ice tray from the freezer to make the freezer and the fridge area bigger.
Remove shelves as needed to make the space bigger.
1. As a heat storage to make the food hot for a longer period.
Place the plastic chopping board or any insulator on the remaining shelf. This is too prevent the shelf from being deformed when a hot pot is placed. The plastic chopping board will melt and get stuck to the pot if the pot is extremely hot. Generally when water or soup is present in the pot it should be ok.
2. To store warm water
Use the remaining space available to keep hot water. Generally, the hot water retain heat better than air or empty spaces.
3. As a dryer or storage
You can use the door shelf as a space for drying or storing cups, food container and etc.