I’ve had mage armour (a pauldron) carved out of a two by four for a couple of years. It’s quite thick, probably at least as solid as plastic plate is, and I sanded it smooth and put several coats of varnish on to prevent splintering. My original plan was just to cut down and pyrograph some cheap wooden dining bowls and plates.
In terms of bones, real bird skulls will get cracked easily, large animal bones less so.
I made a “dragonbone” shoulder piece out of Multimorph thermoplastic. You can get a 250g bag for £8 which is way more than you need for several mage armour bits. It melts in hot water, and I just rolled it out like pastry and cut it to shape with scissors. You can heat and mould it to shape until you’re happy, and then it’ll easily take acrylic paint and varnish.Don’t use spray paint unless you have the flexible stuff, it will crack as this stuff is flexible (you may be able to get away with a single coat as primer, mine cracked and I went over it with acrylic and flexible marine varnish.). There are some excellent cosplay tutorials on painting bone effects.