A parametric spring fit lens cap for an SLR camera.
This thing has been modelled in one piece to make it as print and go as possible. It probably would have been easier to make it out of several pieces that snap together but that just didn't seem as elegant. The springs get printed straight onto the build surface and the main frame has to bridge the gap between them. The clips are held in place with an angled running channel. Depending on how good your printer/plastic is at bridging, the gap parameter can be increased/decrased for a tighter fit. The angle of the slope used in the sliding channel can be adjusted depending on how much droop you get when printing overhanging slopes. The first time I tried printing I produced a rock solid lump so I've adjusted parameters to what seems to work best for me. The 'cutout' STL file is provided so people can easily see the inner workings.
I've generated an STL for most popular lens sizes but I'd encourage people to mess around with the embossed aperture logo on the top and add their own personalisation.(See my other thing http://www.thingiverse.com/thing:13963 for some brief notes on turning images to .dxf format.) You could even fool people with your genuine Nokin lens caps!
Aperture Science logo from aornish on deviantart. http://aornish.deviantart.com/art/Aperture-Science-Vector-Logo-76503583
Like he says: "Portal and Aperture Science are copyright of Valve."