Ya' know, the trick is that either you need a really thick strong material to span a wide unsupported space, OR you need transparent supports. The suggestion to cast them was reasonable, although in my (limited) experience, it's difficult to cast resins with anything like optical clarity, and I for one wouldn't be able to estimate the load strength of such a beam.
I alluded earlier to a torsion box design - it's like a panel with internal supports lenght- and cross-wise jointed like the spacers in old-fashioned egg crates. I've built furniture strong enough to support adults using ordinary corregated cardboard using this method. Look up "torsion box", woodworking" for more info. If you were to use 6 mm acrylic as the skin panels AND as the internal supports/spacers, you'd be able to shoot up through it, although you'd have a pattern from the supports.
Another idea - use segments of clear acrylic pipe as the internal braces. If you take a 6 mm panel, say 1.3 x 2.2 m in size, and literally cover it with 3-5 cm lengths of pipe, then glue another 6 mm panel on the other surface, you'll have a panel about 4-6 cm thick, it'll be much lighter than a solid 5 cm panel, and - if you fitted and glued the pieces carefully - it'll be about as strong as the solid 5 cm panel.
Of course, you won't get any photography done while you're doing all this building.
Good luck, Marco.