I think it is great to design your own controls and circuits. I'd love to do the same, but sometimes I just don't have enough hours in the day, so I run to the nearest electronics surplus store and see what they have.
You may want to look for high efficiency DC/DC converters on line and see what you could find that could be more efficient than this circuit. You can find them on ebay or places like BG Micro or Electronics goldmine, or MPJA. They will usually work with a minimum of external parts.
I also found a 24 to 12 volt converter at BG Micro for $18.08 and it will handle up to 5 amps, and they have a larger one that will do up to 20 amps.
Here is the link to the manufactures product page:
http://www.vellemanusa.com/us/enu/product/view/?id=522479#
You will have to search around BG micro to find the device.(PSDC05, PSDC20)
If you find a high efficiency DC-DC converter, you may also want to find a 24vdc to 5 volts device. Just connect the 5 volts to a Female usb cable and you can charge your USB devices. I bought a set of cables to charge things like my PDA, Cell Phone, Bluetooth headset, etc. No need to go up to 120vac then back to 5 volts to charge a cell phone.
-Chuck