I used a Maxijet 600 when I ran it like I described. I wanted it to be a filter also, so I stuck the filter holder on the inlet, and shoved the CO2 tube thru a slot in that piece. Then I put a filter sponge over it to filter the incoming water. The objective is to have the CO2 go into the water ahead of the powerhead rotor, not after it. The air fitting that you found for the Maxijet is on the outlet of the unit, not the inlet, so it won't work well there, giving much too big bubbles of CO2.

I can see from your photos that you could put the external reactor vertical, but with some effort. It will work a lot better if you do that. Otherwise, the Maxijet method will also work fine. The object of the mist method is to provide gaseous CO2 to the plants in additiion to what is dissolved in the water, because it is more easily absorbed by the plants in the gas form. This method "wastes" CO2, but CO2 is so cheap it doesn't really matter if we waste some of it. The results are spectacular when you use mist CO2 - the tank is just filled with microscopic bubbles of CO2 and the plants are all pearling all day long. This causes maximum plant growth rates which can be a nuisance if you don't want to do a lot of regular pruning. I stopped using that method largely because I'm too lazy to prune twice a week, but also because I had more difficulty keeping BBA under control than with regular CO2 dissolved in the water.