you know about these mattresses that they sell for swimming?
they are entirely made of plastic and could be produced on-site. the advantage is that as the structure size is smaller, you need less material to build the greenhouse, and also since it has a steeper curvature, it can be pressurized more easily (the less curvature, the thicker the materials have to be to hold the pressure within).
to harvest the algae, you'd open the lid on one side and let the whole goo out, including the water, which you then put through a filter to extract the algae out, then you can recycle the water.