Take an even number of two-liter plastic soda bottles, and divide them into pairs. Remove the caps from each bottle in a pair, and position the caps so that one is right side up and the other is upside down on top of the first cap. Drill a hole through the center of the top cap, penetrating through to the second cap. There are now corresponding holes on each cap.
Push a threaded bolt through two holes and secure it with a nut on the other side. The two caps are now bolted together. Screw the caps back onto the soda bottles until the caps are securely in place. The pair of bottles are now bolted together also. Repeat this for each pair of bottles.
Cut a piece of plywood the size you want your raft to be. Lay all your pairs of soda bottles in a line, and place weights at each end so they don't roll away. Position the plywood on top of the bottles. Make a mark with a pen on one of the bottles, then make a corresponding mark on the plywood directly above the mark on the bottle. Continue until there are marks on all your bottles and corresponding marks on the plywood.
Punch a hole in each of the marks on the plywood, then punch another hole one inch away from this hole. Thread a cable tie through both holes, and leave it unfastened. Repeat this with each bottle. Remove the wood, and drill a hole through each pen mark. Replace the wood on top of the soda bottles, and thread the unclasped end of each cable tie through its corresponding hole on the bottle. Feed this through the clasp and pull tight. Repeat this with all the cable ties to complete the raft.