Another strategy I have seen for heat is getting a pile of rocks hot in an outside fire and bringing (tin) pails of them into the tent.
A bucket full of hot sand or stone radiates a decent amount of heat for a few hours anyhow. Just be careful about getting fabric against it. Rocks can easily be hot enough to melt or even burn fabric.
Come to think about it I guess it could give you a nasty burn too if you touch it when it is real hot.