I have a NIB model I am about to assemble and the hard plastic backplate, never used, has fine radial cracks. My friend has two secondhand models, one was remains after the spinner back plate failed. I'm making a pair of alu backplates. I've had a really close look at the cones and they seem OK and are made from a different plastic, I'm happy to use mine. One failure mode that makes it seem that the cone failed is how the cone attaches into the four hard plastic backplate stubs. If one of these strips out or a screw comes loose the cone will be thrown.

If you make a back plate, due to the size of the cone, make sure it can capture the aft rim of the cone. Balance it all and make sure the attach points are solid.