The pop-up hot yoga tents exist and I've used one for two years. It's a zipped fabric dome you set up over your mat with a small heater feeding it, so you're heating four cubic metres instead of forty.
Honest assessment: it gets properly hot, it costs a fraction to run, and it feels absurd. You can't see out, there's no teacher, and the sequence is on a phone outside the tent shouting through fabric. I use it maybe once a week now. It solves the physics and it doesn't solve the reason the class was good.