If you want tapered then openSCAD has scaled extrusion or hulls. I think Fusion has similar tools.
The way I do this in TinkerCad is that I build the hollow first
Open SCAD calls that “difference”, I think fusion calls it a cut.
If you’re working in fusion you usually want to start with a 2d sketch and extrude it. You would make each holder as a component and then attach them later.