Aluminum foil. If you want to keep the heat out it reflects it back out very well. If you want to keep the heat in it reflects it very well. It's not much of an insulator though. But it is recyclable when you need to. Best yet is the bubble wrap that has a reflective barrier built in. It's very durable and works very well. Worked very well for me once in an old almost uninsulated house turned office.
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As the commenters have said, anything which traps a layer of relatively still air will work. For cheap and temporary, I would go with cling film / saran wrap and sticky tape (an extra set of hands to help get it flat would help as well). For a less temporary version, there is a heat shrink film that you can use (random example). For a replaceable method (you can fit and remove the glazing as the weather changes), you can fit plastic or acrylic sheets in place with magnetic strips (another example from the same supplier).
From your link above: "Bubblewrap as window insulation is fantastic" To be clear, it's the air trapped between your window and the plastic that is the insulator. This means that almost anything that can hold the air in the gap will work to a degree. On your environmental point: outside of plastic, your options are: a) any other (semi)impervious membrane over the opening to create that air barrier. b) replacing the windows with double glazed units (which have two panes of glass with an air gap to help insulate) also, expensive Plastic is your best bet for a temporary fix, or a thick rug if you're in a bind. If you're worried about the environmental impact of plastic wrap, there's nothing stopping you from re-using the plastic again next year, or for another project...
Semi-transparent like oiled paper or thin rawhide/fish bladder/islinglass? Cheep depends on where you are and what you have...
Window insulation works by trapping small or thin pockets of air. Air is a poor conductor of heat, so long as it can't move by convection. Double-glazing would work, but would cost you. Cling-film and tape might work if your gap was small. L