I was aware that the Opal Perspex was diffusing the shadows and so bought large (700x1000mm) sheets of frosted and clear acrylic. The intention was to place the blocks and cubes on top of the clear sheet, which sat on top of the frosted one for back-filming, and the whole assembly could be lifted off and placed on cyanotype paper to make a print.
While the strategy for making the print worked (and allowed me to make duplicates of an arrangement), the frosted sheet was actually frosted on both faces, leading to a doubling of the shadows. Additionally, this material was a little too translucent, and provided uneven illumination, quickly becoming darker the further away it sat from the window. The clip below (which overlays footage from days 3, 5 and 6) uses a gradient over the original footage to try to lighten the dark half.