After some thought I came up with a useful, reusable method for easily switching out shaped filters. This method is documented here.
You will need:
* A camera. Any DSLR camera should work; point-n-shoots tend to have quite small lenses so may not work so well.
* A lens for which you know the key measurements: focal length, and maximum aperture. I used a 50mm f/1.8 canon lens. Something with a large aperture relative to its focal length will probably work best.
* Black construction paper
* White glue (Elmer's or similar)
* Scissors & sharp craft knife.
* Pencil, ruler, and (optional) pair of compasses.
* A cheap UV or similar filter to fit your chosen lens (optional) Why? I wanted to be able to permanently stick the filter holder in place without ruining a good lens. I bought the cheapest UV filter I could, about 99 cents on eBay!
Step 1: Calculations
50mm / 1.8 =~ 27.7
I went with a 26mm diameter to be safe.
You'll also need to measure the inner diameter of your filter or lens body.
Ok, maths over, on to construction.