Create a simple solid body turtle shell?

I need to create a turtle shell +/- 20" x 12" x 4" high. It needs to be a solid body so I can add features inside for mechanical widgets. I also need to shell it to what I may need i.e. more or less .5" thick. However, after I create the shell, I would like to create a outer shell turtle look. Would it be possible to create a plane above the shell and create the turtle shell pattern, than cut the pattern by matching the shape of the outer shell?

Thanks
Vince