I have always been unhappy with all those standard 
eye-socket loupes. Too small, too big, they fall off, wrong 
focal length and additionally, they never are where you would 
expect them to be... (colleagues, kids, wife... ;-) 

Here's my solution...

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The loupe consists of 2 parts, the "body" and a "lens fixture",
which holds the glass in place.
A third part "debug tube" is provided to save time and filament.
It can be used to tune the diameter to fit the lens.

Do not print the STL file, this is only an example.
Load the provided OpenSCAD file, modify the parameters and
then compile and export the results.

- measure the lens diameter and change the value of the
  "lens_dia" constant

- set
  PRINT_BODY       = 0;
  PRINT_FIXTURE    = 0;
  PRINT_DEBUG_TUBE = 1;
  to reduce the output to the "debug tube"

- Compile, export, print and check if the lens fits inside.
  If it doesn't: adjust lens diameter value and repeat

- There are a lot of other parameters available,
  adjust them to your needs. Usually, you'll only need:

    eye_ring_dia

    lens_dia ()
    lens_hold_thick ()
    lens_tube_length ()

    lens_fixture_length ()

    eye_lens_dist   

  Values marked with (*) require special attention.


- after adjusting all the stuff, set
  PRINT_BODY       = 1;
  PRINT_FIXTURE    = 0;
  PRINT_DEBUG_TUBE = 0;
  to print the body

- and
  PRINT_BODY       = 0;
  PRINT_FIXTURE    = 1;
  PRINT_DEBUG_TUBE = 0;
  to print the lens fixture