Note: You need to have OpenSCAD installed to use this.

Parameters you can change:

Length
Height
Thickness of top section
Diameter of magnet
Thickness of magnet
Thickness of wall around magnet
 

The supplied stl file is created using the parameters in the included .scad file:

Length = 40
Height = 15
Thickness of top section = 2
Diameter of magnet = 10
Thickness of magnet = 2
Thickness of wall around magnet = 0.5

See the comments after the variables at the top of the file to see what each variable does.

For people who has never used OpenSCAD:

1. Load the file
2. Edit parameters at top of file
3. Press F5 for a preview (holes and filets will look a bit wonky)
4. Press F6 to render
5. Press F7 to export as STL