I modified a remix from hellminor (originally designed by ThatGuys also on thingiverse), to make the shell flat, so kids can play with it, it doesn't roll.

EDIT : added some spikes to be able to have the blue version.
I cut in two pieces in case you want to print in different colors separately. Though I'm well aware the topology is awful, but it's too much work modifying this existing shell.

On the picture with green ans red shells, and blue with spikes, those are hand painted, I don't have multi material system.