There exist already many lens cap holders, but most of them are attached to the camera strap. I found this not so practical, since when I hold the camera, remove the lens cap and try to clip it to the holder, the strap is moving and it's not so easy to hit the right spot. So my lens cap holder is different: it attaches firmly to the bottom of the camera via a bolt screwed into the standard tripod mounting thread. By default, this lens cap holder is dimensioned for my Canon PowerShot SX-40, which has a lens cap diameter of 50mm. The SCAD file, however, can easily be adapted to any other lens cap size. Also, I found that on my reprap printer the thread widths had to be slightly decreased to fit, so I had to finetune it by setting correction term "corr=0.01" in the .scad file.