DIY AC Line Splitter
I have created a line splitter like this with a heavy duty male and female three prong plug and three 14g wires - white, black, and green. 12/2 romex will have a bare copper ground, and I didn't want any exposed wire so I used actual green insulated wire for ground.

Use plugs that have an integrated wire clamp so the connections are secure, and don't leave it plugged in for longer than you need to make your measurements because the exposed separate conductors are not as strong and make a trip or tangle hazard.  This is a special tool, not a permanent extension cord.

I know you said you didn't want a product, but if someone does, there are devices like this that are made to do just what you want in a safe, listed way.  It has one loop for regular measurements and a second loop that multiplies the signal 10x (it just as 10 loops of the wire there).