what a cool idea. I use those bottles for bead storage; cut around 3 sides right where the bottle gets big, and it becomes a box with a hinged lid.

Below is a link to google images of bottle greenhouses; looks like most are round pop bottles with the bottoms cut off, and have been threaded on vertical cords or rods placed close together to make walls.

Seems like the square bottles could be placed on their sides like bricks.

Here is a link that might be useful: bottle greenhouse images

I just fill them with water and use around plants in spring (use as wall-of-water).