We have the same limitation when flying within airport control zones, and I use the Spektrum Telemetry Altimeter.

Set to provide a warning at 400ft above ground level, the system gives a continuous tone or tx shake (or both as required) when the model is flying above 400ft (or your chosen warning height) and stops sounding when the model drops below that height again. To be certain, I set my warning for 380ft. To avoid annoying fellow flyers when the warning operates, I don't use the tone, and just have the tx vibrator sound. This has been working like a charm for me for several years. And that height limitation is a royal pain with any reasonably large model, never mind a jet

Gordon