Desertpig,
The on board devices are useless as they don't put a load on the batteries.
Not so. If you read it at the end of the flght before the batteries build back a surface charge you get a very good indicator of battery life. Because its reading the actual load that was applied during flight. Not a simualated load. Now if your talking about getting a good indication of battery life "Before" the flight I'd also use a loaded voltmeter.
Speaking of loaded voltmeters.
It depends if you want a hand made kludge at the field you have to fumble wth or a compact sealed unit that is simple to use and bullet proof. Yes, there are always ways of doing it cheap. To me $40 for an invaluable tool that I only need to buy once IS cheap.
However if you've already got digital voltmeters buy a dual banana plug at Radion Shack. Solder a #47 bulb for a 200 mA load across it. plug your Hobbico receiver leads into it and you don't have a kludge it's mall and plugs in. You have an indicating load with an accurate voltmeter for $5.00 plus the price of the Hobbico leads. For me this beats having a $40.00 specific built meter that absolutely useless for anything else.
Just my 2 cents.
John