Meshing problem

Dear colleagues,

I'm modelling a thin walled element (solid geometry) and I want to mesh it with an hexahedral mesh with only one element along the thickness. I attached an image of the geometry.
The main problem is the complex geometry with circular holes and curves. I try to mesh a surface with quadrilateral element, but I can't do it on surfaces with holes.
Is it possible to mesh it with quadrilateral elements? If it is possible I can extrude them and obtain what I want.
If it is a problem I'll work with a triangular mesh.

I hope I was clear enough

Thanks for any suggestions
Best regards

Simone