35mm and any other camera with removable lens:
a) Remove lens and tape pinhole over opening
b) You can add cardboard or plastic pipe "extension" tubes if necessary.
Oatmeal boxes and other "found" objects:
a) SINGLE SHOT! and can be loaded with only one piece of film/paper at a time.
b) Make sure it is light tight. A good way to do this is to paint interior flat black.
c) Use black opaque tape to seal edges where box is opened
d) You can produce the most "bizarre" images, because of the shape of the film plane. [not flat!]
e) It does not look like a camera (candids, etc.)
Paint Cans - most hardware stores have these for $1-2 for the qt. size..
A one quart can will hold a piece of film or paper up to 4x10 inches. Use MATT or PEARL paper, GLOSSY causes strange vertical reflections. A #79 drill will produce the sharpest images on a one quart can. Paint the inside and bottom of the lid with flatblack paint. Use a washer to undo the lid afterwards. Make sure the pinhole is clean. Easy to get paint, etc. into the hole.
4x5 see Foam Core 4x5
a) This is one of the most universal film formats, lots of film types available, including Polaroid
b) It can be made from a variety of materials from foam core to wood to plastic to metal.
c) It allows for more than one image to be taken during a session by using the removable film backs and film holders.
d) You can adapt a "real" 4x5 camera easily by removing lens and adding a pinhole to a lens board. Of course, this can also be done with other sheet film cameras from 2 1/4 x 3 1/4 to as large as you have!