I just got this one and it works for me, no worries about wire gauge.
http://www.ebay.ca/itm/DC-0-300V-200A-Shunt-Voltage-Current-Panel-Meter-Digital-LED-Voltmeter-Ammeter-/271729368180?hash=item3f44551c74:g:~~0AAOSwc0FUqBWSI have the one with a 200a shunt, but you could go with the 100a shunt one for the 60 amper.
You use a bit of #4 (or to whatever AWG your max amps will be) between neg battery post and shunt, clamp the charger's neg clamp to the outer end of the shunt and put the pos clamp on the pos battery post. Ta da! There is a recent thread on how to wire the meter to the shunt. I failed that IQ test at first. :(
the 30w marker would be at home though. You need a marker for when to stop the generator when doing 50-90s or 50-80s, whatever your routine is. On the two 6s that would be 10 amps for 90%, but I don't know what the input watts or VA to the charger would be then.
