Customized version of http://www.thingiverse.com/thing:45950

Created with Customizer! http://www.thingiverse.com/apps/customizer/run?thing_id=45950

Using the following options:

whole_periods = 5
fragments_per_segment = 8
height = 6
turns = 3
top_and_bottom_reinforced = yes
rail_diameter = 1
solid_ring = no
mirror_reinforced = no
inner_diameter = 18
segments_per_turn = 50
width = 1