Welcome, Papabugs. A search of the forum should yield some useful information. I don't think there's an aspect of prop building that hasn't been at least touched on in the 11 years HF has been online. Good luck with your research.
If you're a really good artist (or know one) you could paint on canvas.
Otherwise, probably foam is your best bet. If you can get fake bones cheap enough, you could slice off the ends
and glue them onto foam. Or use up some space by making ledges or niches for larger pieces like below.