Both my multimeters will only measure current up to 10A. What kit would you recommend to measure higher currents, up to about 40A?
Build yourself a shunt. Take a piece of #14 or #12 bare copper wire about 6 inches long and connect it to robust terminals (enough to handle your max current). If the shunt gets warm at the currents you work with, it won't be accurate and you will need to use heavier wire.
Then use small wire and tap off the shunt near each end, but not touching the big terminals (like this):
The "#" denotes the large connectors, the "|" denotes the small wire. The "---" denotes the bare copper wire shunt.
Now, connect the small wires to your voltmeter on the millivolts scale and apply 2 different known currents to the shunt and see what millivolts you get for each. Then do a first order (linear) calibration and you have your current meter.
Alternatively, you may want to check out a local college or technical school and see if they have any high current shunts left over from the "good old days". They may have dusty boxes of old lab parts they don't use anymore and they may be able to give you a laboratory shunt (which will already be calibrated for you).