With the disclaimer that we should avoid flames in tents like the plague:

An Ucco lantern is pretty safe, but not much heat - mostly mental.

A Coleman Xcursion lantern gives a lot more heat and light than an Ucco and is not much larger

Beyond that... I've been forced to use stoves in tents this size and have used 2 burner lanterns in slightly larger tents. But I REALLY do not feel that stoves in tents are a good idea in general. If you absolutely must add heat in a tent, IMHO compressed gas devices (stoves, lanterns, cat heaters) are a little less dangerous than anything using liquid fuel. At temperatures much below what you describe, only propane will get the job done, but be careful to keep your gear away from the SportCat.

You could also extend your rainfly to near-ground - it's not hard with a sewing machine. Although the tent will still be at ambient when you enter, it stays warmer. Which is a drawback in warmer weather... I've done the extend-the-fly routine and it helps - even more if you build snow walls up 1/2 - 2/3 tent high and close - keeps the wind from whipping thru.

You could also hang some fabric inside to lower the ceiling and retard airflow thru the mesh ceiling panels. Light weight cotton or ripstop nylon (uncoated) would work if you could get it fastened close to the edges.

HTH. Watch those heat sources in your tent!

Tom