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Mike,
You can use a short piece of 1/2 inch PVC (white or the gray conduit) to fit inside the bottle neck - it seals fairly well as is or with a little tape. The gray coupler is only needed if you're building in 3/4 inch pipe. Not sure even epoxy will stick to the stuff they make the caps out of.
With the 1/2 inch PVC inside the bottle neck, you can use a variety of adapters to join that 1/2 inch piece of pipe to whatever size tubing you're using for the launch tube. See post above for the various sizes you can match to the model/scale you want.
Just for reference, I usually build the launchers completely from 1/2 inch PVC pipe so I can easily slip them into the bottles. I then use a 1/2 to 3/4 PVC adapter to mount a wider "auxillary" 3/4 inch launch tube and a 1/2 to 1/2 CPVC coupler inserted into a 1/2 to 1/2 PVC coupling to mount a narrower 1/2 inch CPVC launch tube.
Yogi
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