I'd like something that's simple and cost effective, DIYer friendly, and passes inspection.
Wait, seriously? Okay then. Let's grab a copy of NEC...
Equipment must be approved by the Authority Having Jurisdiction.
That's the local inspector. But he doesn't run a testing lab, so he relies on Underwriter's Laboratories aka UL or other equally trusted testing labs: CSA, TUV etc.
Essentially none of the stuff sold on Alibaba/AliExpress has a UL listing or equivalent certification. That same junk pipeline also floods eBay and Amazon Marketplace. The latter is your source here. Not everything on eBay and AM is unlisted junk, but it's in the high 90’s.
So you may have a problem with this current sensor being usable. But somebody should make an equivalent sensor with proper listing.
This is driving me crazy here. If you had come in and said "I want to run a 20A circuit with a 12AWG backbone and come off it with 14AWG taps down to each receptacle, coz they're 15A" We woulda said "No, no, no!" But somehow, coming off a #6 branch with a #18 is okay??
Well I think BatsPlasterson has the answer, in his 210.19(A)(4) Exception No. 1(c). It's not a receptacle tap and it's short. However it requires 12 AWG wire at least for the tap. #10 might splice better with the #6, assuming the device can accept #10.
Junction Boxes and enclosing wiring
The illustrations on that Amazon page show just bringing the CT wires and individual voltage supply/sense wires right out of the junction box and onto a piece of plywood, then to the module. You can't do that.
The voltage supply/sense wires are mains voltage, and must be inside cable or conduit. The module has exposed mains screws, so it too must be inside an enclosure.
You'll get major points for a grounded metal enclosure e.g. Junction box, as that will alleviate some of the risk of the non-listed device, "let it burn" inside its steel box. Of course since they didn't use smoke-safe plastics, the fumes may yet kill you.
My recommendation here would be two boxes linked with a short 3/4" conduit. In the left box, all #6 supply wires come through, where they splice to #10? branch wires to carry voltage over to the sensor. The current transformer also goes inside this box. In the right box, you mount the measurement module.
The current transformer needs to come through that same conduit. Low voltage wiring is generally supposed to be 100% separate from mains - however that is impossible with a current transformer! So there is an exception that low voltage wiring can travel with mains if the low voltage wiring is associated with the mains wiring it's traveling with. Which this is.
Further, once LV wiring is in, it's all in - it must use mains wiring methods its entire route, i.e. be in conduit or mains-rated power cable like NM. You can't have a LV control line mingle with mains, exit mains wiring, and travel in cheap 24V thermostat cable stapled to joists. Any short in that box could put 240V on that thermostat cable and POOF! So that is why the CT must go in the conduit.
Cubic inch wise, in the left box, you'll have four #6 wires (5 cu.in. each), 1 set of clamps (5 cu.in. same as the largest wire), 1 ground (5 cu.in. same as the largest ground wire). and two #10 (2.5 cu.in. each). That equals 35 cu.in. A 4" square box won't cut it. I would reach for a 4-11/16" deep box which is about 41 c.i. Buy those at an electrical supply unless you like paying $6 for them at a big-box.
For the right box it's a question of what will physically fit. That's it. You'll have two #10s (2.5 cu.in. each) and some LV wiring so the statutory requirement won't be much.
How are you going to see this thing?
Since the entire unit must be secured inside a mains voltage cocoon of conduit and junction boxes, I fail to see how you aim to actually read the display. You can't just have an open junction box with no cover!
Perhaps this has a head unit that attaches to the main module? I couldn't tell from the pictures. You have to look at its, cough cough, UL Listing, cough cough, and the Labeling and Instructions (which you are obliged to follow, NEC 110.3, since those were the conditions under which it was tested) -- and see whether it is allowable to bring that head unit outside the mains shielding cocoon.
If not, that's a conversation you'd need to have with your AHJ. I would expect a competently made version of this product, built to be Listed, would have a metal case and a small compartment with a knockout for making the necessary connections to Code. This is the kind of thing these very cheap Cheese manufacturers tend to just skip over.