If you're looking for a Centauri Carbon exhaust duct, the nice thing is you don't really need to design anything yourself. There are already several printable exhaust adapters and complete ventilation systems made specifically for the Elegoo Centauri Carbon.
The basic idea is pretty simple. You print an adapter that mounts to the exhaust area on the back of the Centauri Carbon, connect a 4-inch or 100mm flexible duct to it, and then route that duct toward a window. This is mainly something I'd look into if you're printing ABS, ASA, or other materials where you don't necessarily want the printer exhausting directly into the room.
One of the better options I've found is this Centauri Carbon Ventilation System on Printables:
https://www.printables.com/model/1695893-centauri-carbon-ventilation-system
This is more than just a little adapter. It's basically a complete ventilation setup designed around a 4-inch dryer vent hose and a 120mm fan. The creator also provides STEP and 3MF files along with the printable parts, which is nice if you want to modify anything yourself.
There's also a really cool Centauri Carbon Filter and Exhaust Vent Combo here:
https://www.printables.com/model/1318287-centauri-carbon-filter-and-exhaust-vent-combo
I like the idea behind this one because it combines filtration with the exhaust connection. It uses the existing screws on the printer and gives you a 90-degree connection for a standard 4-inch dryer hose. The creator also recommends using an inline fan when venting ABS or ASA if the printer is located in an occupied room.
Another option is a 100mm exhaust duct adapter. These are a little simpler if you basically just want something that attaches to the back of the Centauri Carbon and gives you a connection point for your ventilation hose.
For most people, I think the easiest setup would be a printed rear adapter, some inexpensive 4-inch flexible ducting, and a window vent. You could take things a step further and add an inline or 120mm exhaust fan to help pull air through the duct.
One thing I would be careful about is pulling too much air through the enclosure. Part of the reason you bought an enclosed printer in the first place is to maintain a warmer and more stable environment when printing materials like ABS and ASA. A powerful exhaust fan constantly sucking air through the machine could lower the chamber temperature or create drafts. You really only need enough airflow to keep the ventilation system moving in the right direction.
This is one of those Centauri Carbon upgrades that makes a lot more sense if you regularly print ABS or ASA. If you're mainly printing PLA and PETG, I probably wouldn't consider it a must-have upgrade. But if your Centauri Carbon is sitting in an office, basement, or other occupied area and you want to vent the printer outside, these printable exhaust systems make the project surprisingly straightforward.