One thing it pays to remember that for any decent looking square or rectangular wave, the rise and fall times can be over 21 times the fundamental with many lower odd order harmonics.
For a 120 Hz supply, if the switching via transistors, or vibrator contacts (with the correct snubber caps) your interference, for the most part, is below the radio frequency spectrum (even if the leakage reactance of the vibrator transformer and winding capacity causes some annoying buzz).
So what I'm saying here, is you can have an RF quiet DC,DC converter, if it runs at a low fundamental frequency. But the 75kHz switching supply suggested in post #7 will splatter RF hash across the radio frequency bands an be impossible (nearly) to shield.
Have a look at the low frequency DC,DC converter in this article (called the Omega Device) that generates no detectable RF over 200kHz (because of the low fundamental). Otherwise, the only way to do it is to go for a pure sine wave supply.
http://worldphaco.com/uploads/WORLDFETRON.pdf