Put the /lens on a big sheet of graph paper. set up a vertical screen (white cardboard box) behind it and them used a laser level to project a vertical line into the lens and onto a line drawn on the screen, drew lines on the paper to match the laser line then looked for where they converged. it was all a bit rough.
I made one by splicing loops into a bit of rope (good excuse to learn to splice). Just don't do what I did and get complacent, let go of the camera swimming for the shot in 70m of water - it wasn't clipped on! the worlds fastest decent followed, caught the camera and found the wreck.