If you had this technology to produce giant rolls of environmentally friendly and durable uv-resistant film economically (ETFE comes to mind but I don’t think that’s economical, or environmentally friendly), there are much better uses for it, that can also be less demanding on the material. The most obvious would be to make domed cities on earth or elsewhere or to make greenhouses.
These will also give you a better understanding of the requirements of the material and the structure, if you do decide to go with the original idea.
I’d probably go for a partly (parts being transparent) or wholly white dome over a desertified area and make a jungle or farm inside. A main thing that turns deserts into deserts is the high insolation leading to high Evapotranspiration - Wikipedia, and reducing the insolation through shade, raising the relative humidity, and reducing wind would all help with that.
It would be simplest to desalinate the water using reverse osmosis powered by solar, or you could do solar desalination in the same or a separate dome, perhaps moving the moist air to the main dome where the water in it can condense or precipitate.
Food insecurity will increase due to global warming, particularly in hotter and drier regions, this would help with that, as a stopgap.