The difficulty with the piece of card is lighting it evenly. You're working with 1/3 to 1/2 stop steps in the wedge, and for usable results the card has to be lit very evenly and consistently over the entire area that the lens can see. That's more difficult to achieve than even backlighting for a diffusion material over a smaller area behind the step wedge.

There is a 'danger' that perspex or opal/milk glass will have some colour cast, and I don't know what's locally available in Oz. It might be less of a problem than you think, and is likely immaterial if you're shooting B&W film. If it appears anywhere near 'white' to the eye when you hold it up to the light, you're well within bounds for B&W. There are diffusion gels for theatre or photograpy that are color neutral. Check for a local theatre or photo studio supplier. Google "theatre supplies sydney australia". Lee and Rosco make suitable materials that are inexpensive. The supply store should have small swatch books for you to use in making a selection.

If you can't find a lens hood, just shoot through any size dark box that you can find/make to put between camera and the step wedge.

Lee