I don't remember if this was discussed already, but you could build up to 100 A. Say you had a DMM that you trust to 10A. Take x identical shunts, put them in series with DMM, and measure voltage drop at 10A. Then put them all in parallel, if necessary compensate differences between shunts with a series resistor, and send X*10A through it. Measure voltage across each shunt add and up for total current. Use this to calibrate one or more X*10A shunts or DCCT. Repeat until you're at 100A.