I would build a hollow rectangle of wood, fairly flat, put the bottles in it, attach the open end to the surface of your raft, make one rectangle for each corner.

so you get an enclosed rectangle wich holds the bottles, the "roof" of the rectangle would be your raft.

i would not trust binding them together or glueing them together.

But then it's not really a "bottle" boat anymore. It's half-bottle, half-wood.

It still is, imo, as the main components for buoyancy are those plastic bottles.