Ex: Create a Plastic Seat

Mesh tab > Primitives panel - slant arrow

Change all tessellation divisions to 1

Create a mesh box 100 x 80 x 10

Extrude 3 faces at 10

Extrude left and right faces at 30

Extrude the back face at 90

Mesh tab > Selection panel > Edge

Select the 2 edges on the back extruded meah
Right-click on the grip, select Relocate Gizmo

Snap to the midpoint of the back mesh

Make sure the Scale Gizmo display

Click on the X axis (red), enter 0.7 for the scale factor

Select the 4 edges of the left and right mesh

Relocate the Scale Gizmo at the midpoint on one of the selected mesh

Enter 0.7 for the scale factor

Select the whole mesh

Turn on the Quick Properties
Change the smoothness to Level 4

Mesh tab > Selection panel > Move / Face

Select the back of mesh

Move the back by Move Gizmo 8 units toward back