Unfortunatley, to change DC to DC is an expensive buisnnes. The old way used to be rotary convertors, basically the base voltage drove a motor which worked a dynamo for the output. The other way was a trembeler, that used a buzzing bar. The resulting oputput was ac then rectified. Modern kit uses solid state switching, and they ain't cheap! Much easier to go up the voltage than down. I'm thinking of asking an electronics guru I know if he can design me a circuit, or maybe someone here has the nessacary Vodoo?