Hard plastic, I forgot you were thinking that. I ageree with you about course ground bone. Whole pieces (like wings, or necks) are fine, but not sharp chunks.
When you see the way they eat a whole bone you can see why it is safer that way. I've shown videos of Jennie here, you can see the way she mashes the bone with her teeth, breaking it down and softening it.
The only bone Queen Eva gets currently is Stella & Chewy's (except recently there has been none) Once in a while I will take some off of Jennie's Sunday bone if it seems larger than I want her to have (to prevent constipation from it) for Queen Eva, but I have to cut it up fine for her so far. She won't work at a whole one the way Jennie does, and I won't give bone chunks..