I would suggest a ducted fan on the cover.
Meaning, put a sealed cover on your ATS.
Then drill two 4" holes, and put a short length of flexible duct to those. Like from a dryer.
Put a fan blowing into one of the ducts.
Put a dryer vent on the other duct, with the little flapper that closes off the vent when not in use.
So when the fan is on, it blows in one duct, through ATS, and out the other duct, and out through vent.
When the power fails, or when you want to eliminate cooling, the fan stops, and it effectively seals up the ATS.
A bit complex, but really cheap parts.
I am thinking of doing that on mine.
Another thought is a small cheap 12VDC pump, connected to an old car battery and cheap charger.
It stays on when power goes off, trickling enough water to keep ATS moist, and to help oxygenate your main tank.