The launch tube should be reasonably easy to add to the launcher, be interchangable / removable and be reliable, ie, should stay on the launcher and not go off with the rocket.
The design (for want of a better word) that I have here allows a large diameter launch tube to be added - this providing the best performance improvement.
To make one, you will need . . .
Push one of the o-rings into place at the bottom of the wider portion of the launcher (as in the diagram - right) making sure that it is pushed down evenly. This will tend to make it a little thicker as it is being forced to take on a smaller diameter than it was made for. Next, push in the second one in the same way.
Then, using the pipe cutter, slice off the end of the 21.5mm od PVC pipe so that it is square. If you are using a Stanley knife to do this, make sure that you do a good job of it as this bit is important. Then cut it to length. Next, Push the pipe into the o-rings as in the diagram so that the PVC pipe goes as far as it can - if you managed to get the end square, it should stand up straight.
When I was experimenting with this, I tried out a configuration with one o-ring (just the first, pushed to the bottom of the hole) and a piece of PVC pipe to stabalise the launch tube but the single o-ring failed to provide enough grip on the launch tube and although there were a number of successful launches, several ended up with the launch tube separating from the laucher. With two o-rings, there is enough grip to launch even at 110 psig and as a result, I have never had this happen using two o-rings.
Use (with a H-Base)
With a launch tube, the rocket has a chance to accelerate with very little loss of pressure or liquid - this acceleration using the full bore of the nozzle with the thrust being around half of that from the discharging of water during the next phase of thrust. This means that if you are going to launch using just air, you will see a massive increase in performance and using only a little bit of water (20 mls or so) will act as a lubricant / sealant on the tube resulting in further gains. Doing this, a six year old that can pump to 20 psig with a stirrup pump can effect a launch of at least 40 feet quite easily.