Good day.
I am trying to accomplish something like this https://all3dp.com/2/how-to-3d-print-hollow-objects-simply-explained/

The blue lizard.

I would like to know what is the best way to accomplish this.

Attach is a solid shape I would like to have the triangles wrapped around So I can extrude and booelan diff.
and in the end it should be hollow. (or is there a better way to do this?)

The other question is the same but it should be only happen inside the highlighted yellow shape.(pic attached)

Thank you!