[somenameforme]

The secondary radiation you're talking about is called bremsstrahlung (braking) radiation [1]. This is precisely why water is, to my knowledge, the current leading idea. It works not only as an excellent primary shield but also helps to minimize secondary radiation, and you also naturally have literally tons of it.

As an aside, this is why the original Moon missions used simple aluminum. One of the typical angles people who don't think we landed on the Moon used is that to safely get through the Van Allen Belts we would have needed massive lead shielding on the craft, when in reality it was shielded by a pretty thin layer of aluminum, precisely to minimize secondary radiation. If you put that thick lead shielding on the craft that people intuitively think you'd need, it very well could have killed the astronauts.

[1] - https://en.wikipedia.org/wiki/Bremsstrahlung

[MeImCounting]

Polyethylene is similarly a good shield because its got high hydrogen content so is pretty good at moderating neutrons. I image a polyethylene structure containing a layer of water is probably something like what we'll end up settling on. Its also quite strong and would make decent structural components.