I have thought aout making my own a few times. It is a very simple circuit, the problem I have is getting a case to put it in.

You just need a proximity sensor and a microcontroller. Press the button it sends the signal and starts an internal clock. Once the return signal is received it stops the clock. And you can now turn time into distance because you know the wavelength of the signal.

I am going to make one and I will let you know how it turns out.