Powerstream sells DC-DC convertors for only a little more than you will spend in parts and time. You may want to see if they have something for you.
www.powerstream.com If you are using parallel bipolar transistors you will need emitter resistors to ensure current sharing. Otherwise, you will have thermal runaway. If you really want to build it yourself, and you need more than a couple of amps, there are much better solutions (ie. DC-DC convertors).
