I've had the same problem! My solution was two-fold: first, I really only had the heat problem when the lights were off so, since autoflowers really don't care, I switched from 18/6 to a solid 24. I next encountered the problem as the nights were getting cold and I turned my own heat down when I went to bed (my tent is, weirdly, in my bedroom). I LIKE "sleeping cold" but my babies didn't... so I've had to keep the room temperature up a few notches from where I like it so they'll be happy - and so far, so good.
You don't tell us what temps you're able to get but you've got enough space in your tent to add a small heater... a friend of mine uses a Lasko (designed to be used under desks or something like that) ... small, cheap (?) and readily available. I'm thinking of getting one myself so I can go back to sleeping cold!
Good luck!