The smaller 1" tube coming down is closed on the bottom, open on the top.
The larger 3" tube is closed on the top, open on the bottom.

You may increase or decrease the size of the unit, but you also increase/decrease the contact with the air/water interface.

Note: the device need not be round, it may be thinner reactangle but such shapes can influence the air/water interface and diffusion rates.

I think a simple square with a hook or a floating or suction cup version of the round pH probe ref unit is extremely useful, but it does not require you to keep inside the tank all the time, just when you want to measure the CO2.

Note:
Such units are easy to remove and add back to the tank again!! They also may be moved from tank to tank easily!!

You keep them in a small glass/cup with DI water in it till you need to place it in the tank. Give it about 1-2 hours before a reading is to be taken. More time is better.

The air gap will equilibrate and you do not have to add fresh KH ref solution each time you move it because it's in the inside tube.

Some folks leave their pH probe inside the tank 24/7, but you do not have to, same deal here.

So unlike a CO2 reactor, this need not be an eye sore.

Regards,
Tom Barr