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.
Step 2: Aperture Holder
In one of the discs, cut a 1mm wide slot a little longer than this centre diameter - for my measurements I used 1.5mm either side.
Glue the two unslotted discs together.
Step 3: Make the Insert Template
Next, trace the slot and cut-out circle from the previous step onto one side, then mark and cut out the shape shown in diagram 2 below.
The measurements do not need to be exact, but the width of the tab should be less than the width of the slot, and greater than the diameter of the inner circle. If you happen to make the width too narrow, you can reinforce the template with another layer or two of construction paper.
Glue the slotted disc on top of the (glued) discs with centre holes, being careful only to glue around the edge (see diagram 4).
Step 4: Create Some Inserts
Here are some samples - feel free to use them, or design your own.
Step 5: Let the Fun Begin!
At this point you can glue the filter holder in place if desired, but it's probably better to make a few test shots and adjust everything as needed before committing. Anyway, add any interesting shots you make using this technique to the comments, and happy shooting!