You will need 24 stone door frames or 16 I was building a rex trap until I realized it was to small, 4 stone foundation 6 wooden ramps or 4, 20-30 narcotics, Tranq arrows or darts, a flying creature to ride and then lead it into the trap. Feed it meat or kibble and bam u have yourself a carno.
A simple way to get these guys is to simply distract them with something delicious. Command something tanky (Ex. Carbonemys) to attack them, then set them to passive so they don’t kill the Carno. Then, you can pelt it with a slingshot or bow and tranq arrow until it goes down. They don’t have the biggest torpor stat, so raising it to max won’t take many tranq arrows (5 if it is extremely low level)
Find one and stalk it from a good distance and wait for it to attack something with a good sized health pool such as a stego and while they're fighting pelt it with tranq arrows, it will probably kill the other dino then start to run away, keep up with it and keep hitting it until it's knocked out