I recently did a two Iotawatt install though on one panel (it’s here).
A couple of random thoughts…
First, you are doing really well to plan ahead. The physical install simultaneously easy (in that it is simple) and really hard in that it is always going to be space constrained and difficult to make look pretty.
You have a fundamental decision to make – are you curious and want to measure things that really are not actionable (but are interesting), or do you want to limit your scope (and so effort) to just actionable items.
Range and stove type appliances are good examples: It’s interesting to know exactly how much you spend cooking a meatloaf, but it is not exactly information that will change your behavior. With LED lights the same is true of most room branch circuits (now maybe different if you have a lot of incandescent, or exterior lights). Even a fridge(s), unless its pretty old and you are looking for an excuse to change, it’s unlikely that info will change your behavior.
I opted for interesting – I put in two on one panel, and even then I often wonder about circuits unmonitored even though they are pennies a month.
Decide. The CT’s and units are not that expensive, but the physical emplacement gets harder the more dense you want. Better to do it once than come back.
Next: I would open up that sheet rock (carefully) and see where all the wires go. My GUESS is you will not have room to put it where you show, as there’s likely 40 or so wires bunched up in there going upward, too short to move aside. You may find it easier to go to one side or below (unless by chance a lot of the wires go down?). Sheetrock is pretty easy to fix, before you decide on a box(s) look.
I’d also suggest you MIGHT want to go to two different places for two IoTawatt’s, just to reduce the number of wires that need to come out and go to one place.
Finally if it were me I might not actually put a box in, but a cover. You might just mount some wood in the wall behind the sheetrock to mount the panel through, and not try dealing with conduit. I am not an electrician, but not sure you need conduit if it’s going to end up behind sheet rock anyway, and it might make your runs a lot easier (if less pretty until you put the cover on, thinking like the covers often installed beside whirlpools and such, just a plastic door that pops off but looks finished.