Figure 1 shows the circuit. It's a bare bones twin-tee audio oscillator adapted from a variety of circuits in the last 20 years of hobbyist and ham publications. My wrinkle is that you really don't need to put it on a printed circuit board and enclose it in a fancy case. Mine is built on a scrap of Vector board. See Figure 2 for parts layout.
Nothing is critical about the construction.
Using a snap-on 9V battery connector almost eliminates the power switch. It's been kicking around the shack for better than 10 years. Usually it's used as an audio source for testing homebrew receiver audio stages. The maximum output level is about one volt RMS at 500Hz. My ear peaks at this frequency, so I use it for my testing.