Concept of modeling :) my very few Shortcuts (i have no memory) B=Bridge P= Split K=Subdivide

Split rectangle
Bridge 2 faces
Kill face(s) here one
Subdivide
Shell
Subdivide
Separate all
View Points
MovePoints
Boolean Union
Separate Face(s)
Extrude (or move face(s) + Bridge = only between 2 faces !
Join or Boolean Union
SubDivide
Cancel
Separate face(s) etc...
...
SubDivide

List seems long but it's very speedy! :) And it's a solid so 3D Printable!