Bungee Anchor Widgets

Got myself a Camper Van conversion and needed to stop thing sliding about whenver I cornered or slammed on the anchors.
So here are a couple bungee anchor widgets I made; one is a simple tab on the old seatbelt fixing, the other goes in the slide rails for the seat/bed then twists and slides to lock in. My rails have 20mm holes in an 11mm slot.
The things are in modules in VanTieDowns.scad so you can tweak the sizes for your needs.

You also need to include the following files in the same directory:
thread_calc & printer_settings give you the ability to make things a set number of threads wide using the standard rectangle+2semicircle model for thread_width and build_width calculations. You should combine this with setting the thread_width in your Slicer (print settings/Advanced in Sic3r)
rodsandholes - a set of functions to set the diameter and number of sides of a cylinder to be the correct internal or external diamiter for a rod or a hole
metric_sizes - a set of numbers which