Have you tried concealing things under a couple of inches of a biological material and wrapping that to the body (beer gut, pregnant lady)?
Specifically, I'm thinking a a few large chunks of ham, coupled with some transglutaminase to aid in the forming of a "meat shield".
We didn't try meat due to the mess, but it would almost certainly work as well!
(Technically, in this X-ray energy spectrum, the amount of backscatter is related to the "effective Z" of the material, where Z is the atomic number of the elements involved. Materials made from lighter elements like carbon and oxygen? Backscatters well. Heavy elements like iron? Absorbs x-rays.)
For millimeter wave, I'm pretty sure the teflon wouldn't work though, since the millimeter wave machines are likely tuned for organic makeup rather than just backscatter.