I understand your writing fine.
Here is your homework assignment:
It doesn't appear that you understand what the reactor is trying to do, so I'll try to explain. First of all, you need a source of CO2. You can either use the yeast/ sugar method, or use a pressurized tank. The latter is more expensive to set up, but is probably more cost effective and easier to maintain in the long term. The yeast method may not be able to provide adequate levels of CO2 in larger tanks.
Now that you have your source of CO2, why not just let it bubble into the water? The answer is, in order for it to have the desired effect, you must make sure it dissolves before it escapes into the atmosphere. The reactor you are inquiring about does just that. Basically, the tubing that carries the CO2 enters the reactor into a small hole in the "sturdy tube". It is best to drill a hole in the tube about 2 drill sizes smaller than your tubing. Then, cut your tubing at an angle and pull it through with a pair of pliers.
Basically most foams would serve the purpose here. The foam doesn't allow large bubbles of CO2 to pass, in other words, the design ensures that the CO2 stays in contact with the moving water long enough so that it gets dissolved. Of course, it's probably best to use a foam that is designed for aquarium use. You could try some of the foams that are used as filter media.
This is an internal reactor. Some of us also use external reactors that are arranged inline with the outflows of cannister filters. Do a search for "reactor" on the forum, you'll probably have a day's worth of reading before you decide on a particular design.