If i can input what i generally use when using external battery power with good results, you may pick anything useful from it.
(Note - the only failures i have recently had are with 'off the shelf mains supplies' designed for cameras so i don't use them anymore, they often don't cope very well with the inrush current during start up).
I use a pre-built LM2596 DC-DC Buck Convertor which are adjustable using a 10 turn built in potentiometer. (easily found on many auction sites, typ £2 each so its really not worth the time building anything yourself).
They accept a wide range of voltage inputs from 4-35v and will output a steady 1.25-33v.
In my experience they do require a differential in/out voltage of at least 1.5v to function reliably.
They deliver a constant 2A or 3A if you fit a heatsink (i have yet to notice any temperature rise during use for my configurations).
They are very small and robust. (zero failures in a year).
I use them typically from 12v input (lead acid), or 10.1v input from li-ion and have Powered A530, A470, A1200, A1300, A2300, Ixus105, and Big DSlR's 600D, 60D and 10D .
My typical configurations to supply these modules are:-
1 Big cheap Mains PSU DC output (5amp cctv units at 12v). good and reliable for test purposes in the workshop.
2 Lead acid 12v battery only. (have historically connected to car for carlapse)
3 Lead acid 12v battery with trickle charger attached (fail safe option).
4 Portable Li-Ion battery direct.
All input to the LM2596 module and set to my desired Output voltage precicely. I find them pretty close to 100% efficient. (connect them to a bench psu with an accurate volt and ammeter and you can see what the whole system draws when idle, taking pictures, zooming etc, and most importantly when you turn off the camera. my digital bench Psu ammeter (00.00) accuracy measures zero when camera turned off, so the LM2596 has no noticable current draw itself.
Example of A1200 is around 25-40ma when saving a Picture or zooming, and typ 10-13ma idle.
I find the A470 and A530 more efficient overall as when idle they draw less than 10ma. (470 is typ 4ma)
Have been Powering full sized DSLR's with these units for about a year now with no failures.
They are easily modified with a rotary switch connected to fixed value resistors in lieu of the potentiometer if you want an exact switch setting to set an exact voltage for a bench test unit with quick predefined voltage set.
Incidentally, out of convenience, i am currently testing my time lapse rig development from my workshop PC 12v output via one of these modules.
Have even had success connecting a module into usb power (5v) and powered my A530 for a 24 hours test. (pretty inneficient Psu i know but if its all you got at the time use it).
Hope this may be useful