So, to get the best hex nut capture socket fit, I wrote a parametric OpenSCAD script to produce a simple test part.

Any screw size with either a hex head or a round capped head can be configured to produce a test part for 3d printing.

Use the Thingiverse Customizer to produce custom STL files.

The project with variable setting details are published here:
http://airtripper.com/1594/hex-nut-capture-socket-sizing-for-3d-printing

Full instruction are here:
http://airtripper.com/1594/hex-nut-capture-socket-sizing-for-3d-printing

Download the OpenSCAD file or use the Thingiverse Customiser to get the required STL files you need.

// - Select part to view/print from list
view_part = 1; // [0:design, 1:head_up, 2:head_down, 3:horizontal]

// - Screw head diameter - flat side to flat side on hex nut or bolt head
head = 6.75;
// - Screw head type - [6] for hex head and [50] or more for round head
type = 6;
// - Screw size diameter
size = 3.65;
// - Screw size diameter smoothness
smoothness = 50;
// - Screw size length - not including screw head
length = 5;
// - Screw head length
cap = 3;
// - Block border thickness between screw head and block edge
border = 4;
// - Print layer thickness - only required for head down print
layer = 0.25;