I've been having great success with cuttings but noticed the fast root growth like I had in the spring and early summer has slowed down. 

My guess which might be 100% correct is because we have the AC going on all the time. Now when I look at my containers I don't see condensation like I used to on the sides.

My question is has anyone put their closed rooting containers in their hot attic or a very hot outdoor barn?

I keep mine outside under a large table for shade. It's so warm outside that condensation and humidity is well-maintained. Roots are developing just fine.  

I have left mine in a really hot shed outside without any problems! indeed, they rooted quite quick and had great success with that!
However, please note that real hot temperatures also mean lots of condensation/evaporation. Must check for moisture evaporating quick and drying up the cuttings if they are not 100% sealed.
good luck!

Hi,
As long as you're full shade you should be fine. Then harden them a bit each day and if they wilt then back to tale shadow.
I had a pot in the tomato greenhouse and with the crazy temps we've had ( 42°C) ... and I missed one watering ... Well the top of the new stems seem to be lost.
Never mind, it is a guinea pig - and the mother tree is still due to ripen a fruit for now - but drops all of the brebas so might be "French Ronde de bordeaux".
I'm waiting on fruition to confirm or not .
For sure don't drag your pots out in full sun, and keep up with your (new) watering schedule .