Another simple and cheap hack is to heat the tent with 5 to 10 well-heated stones (the bigger and smoother, the better). Lay the stones 5 to 10 inches away from the campfire to heat the stone gradually. Turn the stones around so that they get adequately heated at all angles. It should take 1.5 hours to be thoroughly heated.
As it does not retain heat as long as bottles, wrap the stones in several layers of aluminium foil to cool down the stone slowly. Place it in a pan, and put a layer of protection under the pan. Remove the foil layer by layer when it starts to cool, as the heat it retains in the air between the foil layers.
Another trick is to dig a one foot deep hole. Place the stones inside and cover them with sand. Then you can place your tent over the hole, which will heat up due to the heat in the rocks for the rest of the night. Do use sand, instead of soil as you want to let the temperature rise smoothly.
Alternatively, you can wrap the stones in a cloth or socks, and spread them at strategic points around the tent.