I designed this as a way of adding a Neutral Density (ND) filter to the front of the P6*6 Pinhole camera.
http://www.thingiverse.com/thing:157844

Decreasing the amount of light exposing your film is a handy way of slowing down an exposure in very bright light. A "fast" pinhole exposure might be 1 or 2 seconds long. At these shutter speeds, the slight camera shake induced by opening and closing the shutter can blur the photograph.

ND filters are made in whole "stops". Further, each stop halves the amount of light reaching the film. An ND2 is one stop, an ND4 is two stops, and an ND8 is three stops. An ND 8 filter allows you to make an exposure metered at 1 sec with a shutter duration of 8 seconds.

Additionally, color filters may be employed for special effects.

The included STL fits a nominally 52mm filter with an actual outside diameter of 54.50mm. Yours are probably different, but you can simply change the parameters in the script. I plan to port this to Customizer in the future.

This parametric design can be adapted to other cameras, like a Holga, and any filter size.

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The OpenSCAD script employs some cleverness to ensure that the larger diameter (filter of lens barrel) is oriented upright, to eliminate overhangs.

I find that my 3D printer doesn't print inside diameters as precisely as outside diameters. Before printing a batch of adapters, use the test_ring.scad file to determine the "fudge" variable in the adapter script.

After all that monkeying around, I found that pressing the filter into the adapter stretched the ring a tiny bit and made the friction fit on the camera a bit loose. I got better results reducing my "lens_OD" measurement by 0.20 mm.
"We must be cautious" - Obi Wan Kenobi

The script is well documented, but please post any questions you have in the comments!

Printed in black ABS, 2 perimeters, 50% infill, 3 layers top and bottom. Sliced in Slic3r Ver 9x.