Well, I did caution you against going down this Atmos "rabbit hole". I take it your attic is full of blown insulation as is mine. I installed the Atmos boxes before the insulation was blown. If you are going to open up the ceiling, someone needs to go up there and move the insulation out of the way, or you will have real mess on your hands. I think it a bad idea not to enclose a driver and have it exposed to an attic space. If you mount a speaker and box on the ceiling and NOT in it, it will look like H. Gear that does not look nice has zero pride of ownership.
I bet you have a good C & C shop that would make your back boxes in no time if you design it. Since I was busy and had no workshop while the home was under construction, I had these back boxes made in a C & C shop, which they did literally in minutes. I had them build the four enclosures for our Great Room in wall system. I arrived at 8:00 AM and I left at 11.00 AM with 7 speaker enclosures, made perfectly, for a very reasonable charge.
In your case I do think you need to have someone clear the insulation before cutting any holes. You need to run the wiring, which should be put in conduit.
Once the boxes are mounted you can connect and mount the speakers, and then place a grill over them. Attention to detail is alway important, and after you have spent what you have, you need to do the job properly or scrub it. Only you can decide if two Atmos speakers are worth the trouble.