You want to turn your SLR or DSLR into a pinhole camera? I picked up one of these cheap Holga HPL-P lenses for my Pentax cameras and it works wonders. The HPL-X lenses also exist for Canon, Nikon, Olympus, Samsung NX, Sony and Panasonic cameras. They’re easy to use, at Ø=0.25mm and 28mm, it has a corresponding aperture of f/112.
Finding the correct exposure can be a bit of trial and error although it should be easy if you have an external lightmeter and bracketing will certainly help to narrow it down. If you shoot film I would certainly recommend a lightmeter and the use of this table to find the right exposure.
Note that even though these lenses are advertised to be usable on SLRs as well, do expect a lot of vignetting as the lens will not cover the whole negative and is clearly designed to work on DSLRs with cropping factors of at least 1.5.