If you are winding the toroid yourself, or if it's not already insulated, I'd fully encapuslate it with epoxy, RTV, or plasti-dip, or something. Then, once you've got the hot wire inserted through it, use some zip-ties and/or heat shrink to make sure it stays in-place over the insulated part of the hot wire. Basically just double or triple-insulate everything
Is that white wire at the top or bottom of the box? Personally I'd put it in that corner area, then route the wires from the coil via one of those nearby smaller knockouts, with the wire surrounded by some spiral loom for extra abrasion protection (CYA mainly); securely attach the coil (wire ties?) to the white cable.
What does your insurance agent say?
because there is some regulation on including low voltage wiring with high voltage wiring (though I think it mainly applies to conduit runs and the like?). You might want to ask around, it if is a concern...?
Bottom picture : The braker box. Big black wire : 120 into 100 A braker, big white wire, Neutral / GND
Measuring only the neutral wire will only tell you the amount of current being drawn by all the 120vac loads.
Never forget the lead-in wires are always energized, unless you call your power company to disconnect (yank the power meter).
it could be fitted downstream of your main breaker (presumably you have those over there) and be wired up in comfort.
Might be a tad more complex with the funny 240v/120v split systems you employ that side of the water.