DIY ammeter for measuring currents as low as 0.1mA
Since you don't care much about voltage drop you can use a resistor and an ordinary 3-1/2 digit  multimeter with a 200mV FS range and 10M\$\Omega\$ input resistance.
A 1K resistor will drop 100mV at 100uA and give 100nA resolution (the display will read directly in uA with the decimal point in the correct position).