I designed this carrier/enclosure for a compression spring to convert it into an extension spring, and protect/retain it safely. This can be used in multiple applications, but my initial goal was to create a way to secure COVID masks without elastic, using normal string, like paracord or shoe string, and not simply use a bare extension ring that can become entangled in the hair. The spring I used is a 23/64" x 1-3/8" (9mm x 35mm) compression spring found in a Sontax 200 Piece Spring Assortment. It is printed laying on a 45° angle with one long edge facing the buildplate and pause at layer height should be set in your slicer to stop the print after the internal pegs have printed but before the opening closes too tightly to insert the spring. This is print-in-place, so the spring is inserted during the print, just be sure its high enough that the nozzle won't contact the spring when it resumes. Also, I used a raft, because of the small surface contact with the build plate and the likelihood of breaking the print from the build plate during spring insertion. The clearances are tight but the halves separate and slide relative to one another easily with the gentle help of an x-acto knife and prying instrument once the print has completed.