Good question! Had to look it up in the paper, and it turns out they use wood screws. Either regular ones using a long screwdriver through one of the bottle's

Good question! Had to look it up in the paper, and it turns out they use wood screws. Either regular ones using a long screwdriver through one of the bottle's mouths, or more specialized double-ended screws (with two points) that let you grab the bottles and counter-rotate them to drive in the screw in both bottoms. Neat, I hadn't thought the bottoms of PET bottles had enough material to make this work, I would have expected bolts with nuts, i.e. more metal-to-metal in the fasteners.