UPDATE: V2 has better offsets, better reinforcements and faster print time (less material). Print on back, with vertical prongs laying on print bed, with supports.
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.
Because this is going up at such a late date, you'll want to choose whatever settings will allow your print to complete quickly while supplying sufficient strength that 2mm arms won't snap on you. 15% infil worked fine for me but you may be able to get away with as little as 8%
Once you've printed the bracket of your choice, to match the tripod or mounting system you have most readily available, and printed one of the NASA pinhole camera state-shaped plates, just slide the plate into the top arms of the bracket. It should be clear from the design but two of the arms go behind, and one goes in front of your pinhole camera plate. The spacing is 2.75mm, so there will be some tension, hopefully sufficiently to hols the plate in position, as the plate itself is 3mm thick.
Obviously, you can just as easily slide a piece of cardboard into this bracket. If you go this route, know that your best bet is to punch a hole 5.5mm in diameter.
Remember, if you don't have suitable Eclipse Glasses, stick with the pinhole camera and view the resulting image only. Regular sunglasses are are insufficient!
Happy Eclipse Watching!