Designed for bottle caps that are ~33mm in diameter and ~7mm height. Diameter must be over 20mm, otherwise the object may get lodged in the carrying rails.

These parts are easy to print, and do not require any supports or post-processing. Simply screw in the spring and snap-fit the case parts together. (I suggest glueing the case halves)

Hardware:

2x M3*4mm Hex socket machine screw
2x M3*8mm Hex socket machine screw
2x Extension spring (45mm unstretched hole-to-hole distance, 5mm diameter)
 

I suggest using a light oil, especially on the thin internal rails to prevent friction.

Also make sure the part is oriented at a 45 degree angle on the buildplate so the print lines are prependicular to the direction of motion of the slider. This significantly reduces friction and noise.

If your slicer has the Polyhole option, make sure to check it. That will make it easier to screw into the material with a stronger hold.
 

• For a weak spring:
  All parts can safely be printed from PLA. Make sure to have 4+ walls for everything. If the slider is made from PLA too, set its walls to 99. Otherwise, ABS or ASA is recommended. (20% infill)
• For a medium strength spring:
  I suggest using ASA/ABS for everything. Print with 4+ walls (5 recommended) and 20% rectilinear infill
   
• For a powerful spring:
  Use ASA/ABS, Polycarbonate or Nylon for the print. Especially for the slider, which will experience the most force. You can mix and match materials. E.g Nylon slider with and ABS enclosure parts.
  6 walls for the enclosure and 30% rectilinear infill, 99 walls for the slider (to make it solid). To prevent the machine screws from ripping out (in rare cases), it might be worth putting some CA glue (superglue) on the threads before screwing in.