I have a setup where I regularly record piano videos to track my progress and I always forget to switch off the little camera pre-amp when I'm done, so more than once I found myself unable to record and with a pretty expensive battery drained flat.

So I made this little box, it takes the business end of a 9V battery and contains a small 7809 based voltage stabilizer. I used an old 12V brick to supply power, the extra regulation helps to avoid line hum. Capacitors across the in and out line (10nf and 100uf) ensure that the 7809 doesn't turn into an oscillator.

It's pretty effective and pays for itself at the first battery. In the photograph you see the original that came with the pre-amp and this version side-by-side.

Material and other details aren't all that critical and if you use 9V batteries in other devices you might be able to customize the box so it fits, I've provided the OpenSCAD file to make that a bit easier.

I've used some hotmelt to keep the cable from being torn out, as well as a knot in the wire, together those provide some strain relief, but that could probably be improved by using a proper strain relief in the hole.