Figure a way to make some sort of control box on top of the outer tube. I used sections of a 1 inch square pine dowel. I epoxied it to the top and reenforced with more fiberglass and felt.
Once the epoxy cures, paint your choice of colors.
EDIT: Picture 2 & 3 - The Locking mechanism (13 seconds into the video)
This is a strip of plastic glued to the side of the launcher.
Nothing special needs to be purchased. Look around your house/environment for a strip of plastic at least 1/8 of an inch thick and about 3 inches long. It needs to flex without breaking. Look around and use creativity and imagination.
Drill a hole on one end for the screw in STEP 6 to go in to. Glue the strip of plastic to the side of the tube in such a way that when the launcher is extended the screw goes into the hole in the plastic. put a layer of fiberglass, felt or duct tape to make sure the plastic does not come off or shift (see the green square in picture 2). a flexible strip of metal will work too.
Edit: People still don't get how to make locking mechanism. I still get messages about it. I think the trouble is people are trying to make it more complex than it really is. The rule is K.I.S.S. Here is a video of the locking mechanism.