Netgeek wrote:
This may be slight overkill (they claim it's good for 100W) but it's
inexpensive:

www.oatleyelectronics.com/kits/k168.html

Alternatively, you could put together something quite simple using
an MC34063A (try Google) - easy to use, readily available.

That unit could run cooler or produce more output if the rectified
secondary were stacked on the +12 input, so it had to produce only the
additional voltage. Just cut the secondary turns almost in half.