How to set yogurt in the room temperature between 20 to 16 degree Centigrade? It is winters here, and the yogurt never sets the way I used to set it during summers.

If for example, we have 1/2 litre milk, what is the near exact temperature (for heating the milk), and the near exact yogurt to be added to the milk for setting?

EDIT 1:
When I mention heating milk, I mean that milk has been boiled quite a few hours ago, we just need a warm up session now. So how many centigrades are required?
You can make the yogurt in an insulated container, like a thermos bottle. A wide-mouth bottle will probably be best. Pour hot water in and let it sit for a minute or two to warm up, then pour it out. Heat and add your milk and starter culture as usual.