I believe you drain yogurt to make yogurt cheese.
You shouldn't have to drain the yogurt to get it thicker.
When I made yogurt I used 2 tablespoons yogurt starter in about 3-4 cups of milk. I would put it in a pyrex dish that had a glass cover. After I dropped in the 2 T, I used a spoon to stir the yogurt-milk mix once or twice. I kept it in a gas oven and it took from overnight to at most 24 hours to produce thick, rich and creamy yogurt throughout the entire pyrex bowl. I did make sure that my starter has active or live good bacteria like acidophilus or etc. in it. I would basically only use Regular Dannon yogurt--non-flavored.
Also, the older gas oven had a constant warmth to it, with the pilot light on. The constant warm temperature is the key here. Some of the newer gas ovens are cold, these would not work. In that case use a dehydrator and the low dishes made especially to make yogurt in them or get a yogurt maker.
hth,
Cathy S.