Martin - your mention of foamglas put me in mind of a product that is/was used as a polyester filler, hollow glass spheres. Can't drag the memory back, but it might have been called something like "microspheres"? Poured like dry sand, and got everywhere when I tried it about 30 years ago !
Obviously a possible low density substitute for the sand rather than for macro inserts like your foam blocks.
John
It's like doing jigsaw puzzles in the dark.
Enjoy today's problems, for tomorrow's may be worse.