One thing people in the Electronic world over look are the ammeters used in commercial heavy equipment world. There you can get meters of 500 amps, or more. should you wish they are designed to a 100% duty cycle they have heavy duty shunts they read from. I know truck drivers who are hamradio operators they use heavy duty ammeters to monitor the power consumption, and charge state of the batteries used to power the 1500 watt ( or more ) HF amps they use in their mobile HF. setups.