Happened to come across this.

Has anyone tried this out?

 

I've used TI's simple switchers and they do work.
I've also used this circuit:
It's a Cuk converter, and its big advantage is that both input and output current are continuous, so it puts a minimum amount of interference on both its output and your existing positive supply. V1 is a square-wave at 40kHz to 200kHz, and for a 50% duty cycle the output voltage will be pretty close to (minus) the input voltage. Provided that there is enough load on the output to keep it in continuous current, it doesn't need any feedback.
You can wind both inductors on one bobbin, or there are some useful surface mount dual inductors which are ideal for building it.