[lindboe]

Depends on the definition of "simple", imo. The first thing that comes to mind is research into materials with good (tens of dB), wideband absorption in the mmWave bands. It's an area of active research [1] [2] (just a couple articles from a quick google, so caveat emptor).

[1] https://www.cambridge.org/core/journals/international-journa...

[2] https://pubs.rsc.org/en/content/articlelanding/2022/tc/d1tc0...