Bob La Londe wrote:

Everybody else mentioned foam, which can soak up moisture
and is difficult to work with.

You can do hand layups with honeycomb core materials like
Plascore and NidaCore. The reps from those companies will
tell you vacuum bagging is required, but it isn't so.

I've built with hand-layed Plascore several times. It's light,
it becomes stiff (with sufficient glass) and it will not soak
up water, no matter what.