This is one way to make a cheap DC/DC variable voltage supply. Its worth watching (not too many mods you need to do). Author does a good job pointing out that you want to check the caps being used that they can handle the higher voltage.
http://hackaday.com/2015/08/13/crack-open-a-usb-car-charger-to-make-it-a-variable-dc-dc-converter/