Printing

PLA is ok option, but you should consider infill pattern, that is supportive to all directions for example Cubic, higher infill percentage (40-50 %) and definitely at least 3 perimeters, but more should be even better. On my first print it was used 15 % infill, which obviously wasn't strong enough, because it broke. Also thinner layers should help with strenght.

Assembly

You will need few extra parts:

• 3x longer nails with diameter about 3 mm
• 2x shorter nails with diameter about 1,5 mm
• Wire or string
• PV (pestra valve - 6mm) 
• Piece of rubber for sealing (i didn't have one at that moment, so its not in photos)

Start with putting PV into the Rocket launcher (stand). I personaly left about 2 cm in diameter of rubber around the valve itself, so it makes nice seal. Then you should glue the rubber on the cap (It is possible, that your sealing will be thicker than it should. I that case, heat your shorter nails and make hole bigger. 

Through the holes on the bottom of the launcher, you can connect whole system to the ground. With the other two nails you should stopped bottle from flying away earlier than you want. It is good to connect them with the wire for symetrical launch. 

video - https://drive.google.com/file/d/1gYoKRPizBfB4HiY0NcI25djBQUVwq5ln/view?usp=drive_link