Make your own!
Here are a couple of examples I have made and used that work well (I did not design them).
The first is for 3 cell (NIMH) HLG - uses only 4 parts but they are more expensive. Can be used for 4 cells and lipos too but you will need to change the value of the resistors and I do not have that data (someone here will probably work it out).
The second has a higher component count and may need a circuit board or vero board, but can also be direct wired together and encapulized in epoxy. It has a 4.4volt cutoff.
Both sound a piezo buzzer (take off an old computer motherboard) when they reach their cutoff voltage. BUT via the capacitor, they also can sound off with vigorous movement of servos as they temporally load the battery on startup. The caps as shown allow intermittent sounding as the battery nears drained. A higher value cap will limit this tendancy. So if the model is lost you can wander about waggling your controls for some hours and getting intermittent beeps.
They are very light and only use a couple of milliamps in monitoring mode. I guess you could also adapt them with another resistor to flash a powerful LED. All parts available from RS Components. The National TC44 is the most expensive part but is used in most portable devices like cell phones to monitor batteries.