This is an expandable, wall-hanging mail sorter I made in Tinkercad for my family. It's very basic and could use some refinement, but it works as is.
3 Files - Baseplate, Top & Bottom Cap, and Shelves.
Each baseplate gives you 4 mail slots. Top and bottom caps fit onto the top and bottom of any baseplate. No adhesive necessary to assemble - tolerances are very tight - just hit connecting joints with a little sandpaper and they'll fit.
If you want to make it easier on yourself, you could widen the grooves in the baseplate a bit or decrease the thickness of the shelves. Currently both are set to 2mm, and that works - but it's not easy.
The dovetails are a friction fit as currently sized.
Screwholes are sized for M3, but it's lightweight enough that I just hung mine with some of the removable 3M adhesive velcro.
I printed from standard PLA using the default settings in my slicer. Nothing special.