That plaster jacket method is a lifesaver for tricky soil. I had a similar thing happen with a bison vertebra in Wyoming, cemented in caliche. We ended up using burlap strips soaked in plaster of Paris, building it up like a cast. The key is letting each layer dry before adding the next, otherwise the whole thing just slumps. It looked like a weird lumpy pillow when we were done, but the bone was perfect inside.

Totally blew my budget doing that last year.

Wait, is it really that big of a deal everywhere though? I get it in a wet climate like Michigan, but what about a dry basement in Arizona? Feels like a one-size-fits-all rule online. Maybe those guides call it optional because it depends on your local ground and climate. Still, for a couple hundred bucks, I guess it's cheap insurance if you're not sure.