There are many easy ways to make a small greenhouse. U's, to create the structure of the greenhouse. Use a couple of metal pipes or poles and lash them towards the top of the structure to keep it immovable. Plastic cable ties are good to lash with. For a larger, more substantial greenhouse, you might use large old windows from a used building supplies source and build a frame for them to attach to.
Green ones are just fine.
Now stuff the whole end piece back into the bottle upside down as far towards the front of the bottle as you can. Do this three times so that the openings open right above the place where the bottom is stuffed towards the front. Cut a square or circle from some screen material and place it over the holes. This will keep the stones or balls from blocking the holes and causing algae to grow in the bottle. We may be able to eliminate the whole end piece steps.