Why not keep it simple? (and cheap), all these shunt and clamp solutions require an opamp to amplify the low level signal, so why not use an opamp to amplify the voltage drop across the existing cable?.
This is just a variation on the shunt theme, but it gives zero loss (the loss was already there) and is easily calibrated using a known load.
This method was used decades ago in EPE, for a car ammeter - measuring the voltage drop (in either direction) across the earth strap to the battery.