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I went to Radio Shack and got a little project box, some banana plugs and receptacles, some wire, a momentary pushbutton switch, and a potentiometer rated for 0-50 ohms and 5 watts (IIRC), plus a knob for the potentiometer. It was easy to drill a few holes in the box and solder up the circuit.
I plug a charging cable into the unit, which itself plugs into a cheap voltmeter. When I push the switch, the load is applied across the Rx battery. Otherwise, I'm reading the open circuit voltage. I calibrated the unit by measuring the resistance with an ohmeter, and I made little marks for (nominal) 250 mA and 500 mA settings for 4.8 and 6.0 volt packs. Total cost was about 5 bucks, minus the charging jack.
Make sure the potentiometer is rated for the power it will dissipate, otherwise you might smoke it if you keep the button pushed for a while.