UPDATE: V2 has better tollerences, is stronger and uses less material/prints twice as fast (Print on back with vertical arms laying on bed, with supports, 8% infill)
NASA has published a nice set of state-shaped pinhole cameras meant to be 3D printed https://eclipse2017.nasa.gov/3d-stl-files unfortunately the meshes aren't all well formed so I recommend you preprocess the meshes through MeshLab http://www.meshlab.net/
The other deficiency which I'm attempting to address here is lack of mounting brackets. I only have two hands, so need to be able to mount my pinhole camera on a tripod, so I threw together this bracket. Just slide whichever of the pinhole camera plates you've selected, into the arms of the bracket, screwing the tripod mount onto your camera tripod, or if you have a gopro mount laying around, I've included a version with GoPro style mounting arms. I realize this is a bit late, as you'll need to print and test this tonight (the night I publish these models) in order to make use of them tomorrow, August 21st the day of the solar eclipse.