We’ve had similar stretches and I wish I’d left even more ventilation space. The coop gets unbearably hot during the day, hotter than the outside temperature, and retains the heat at night, taking longer to cool down than the outside. What’s been helping in my case is that for the last hour or so before the chickens go to bed, when it has started to cool down outside but it's still hot in the coop, I lock the chickens out of it (in the run) and open the human door all the way. It sits like that for an hour and it really makes a difference to cool it down. By the time I let them back in, it’s the same as the outside temperature, and then continues cooling down throughout the night. In your case, you can try opening the access door, and also the nest box lid, anything that can open, to get some breeze going through in case it's retaining heat. If it's the same temperature as outside, though, I don't know what else you can do...
But 78 is not bad at all. Even if they're panting, if it's really just 78 then they'll be fine. Might be because they don't have enough roost space and sleep packed too close, keeping each other warm. Or maybe it's an individual thing. Are they all panting, or only some of them? Mine have two main roosts, one lower than the other, and they technically have enough room if they spread out, but they all insist on sleeping on the top roost, even if it means cramming close together and staying hot (this is a temporary situation until I cull, then they'll have enough room on top). They'd rather be hot and pant, than endure the humiliation of the bottom roost

And then there's individual variance. I have this one chicken that always pants no matter what. He pants when it's 72 degrees in there. He's big, fat and fluffy though, so probably overheats more than the rest, even from the same breed. Used to stress me out, but now I just ignore him if he's eating and drinking and otherwise acting normal. We even gave him a name that's a derivative of "open mouth"