I'm keen to observe the Transit of Venus and I am even more keen for the Junior Grendel's to also be able to do this, but I do not have access to the solar film I need for my telescope. Instead I have decided to make a pinhole viewing device that will project the sun onto a piece of paper so that they can view it safely.
I found great advice here http://www.exploratorium.edu/eclipse/how.html by Ron Hipschman as well as this site at Stanford Solar Centre that recommend a variety of ways to observe the transit: http://solar-center.stanford.edu/observe/
Based on what I have learned I have decided to make each of my boys a viewing tube to take to school. To keep it simple I will use PVC pipe that I can cut to a good length but that will project a reasonable sized image. At one end of the tube to place a piece of alfoil, and as close as you can to the centre you create the smallest, smoothest pinhole that you can - the smaller the aperture the sharper the image of the sun that is projected will be.
I am using rubber bands to attach the foil so that if it gets torn or crumpled I can replace it quickly.
At the other end I will attach four wire braced to hold a piece of white card to project the image on to - and to prevent small heads from looking through the tube directly at the sun - always a terrible idea.
The length and diameter of the tube are important as the longer the tube, the larger the projected image will be - but, if you make the tube too long the image will be wider than the diameter of the tube and you will not see the edges.
The formula to calculate the size of the image was one I found here and is fairly simple 0.0093 x Length of the tube. A tube that is 1000mm long will provide an image that is 9.3mm in diameter, so using PCV pipe for water rather than electrical cabling will likely ensure your image does not touch the sides and will also be less likely to flex.
I'm going to use 80mm PVC pipe for this task and probably try to make my projecter 2000mm long to provide a viewing image of about 2cm in diameter.
Construction day is tomorrow so I will provide instructions and pictures based on what I learn.