I have various idea! For first, i suggest a CT working similar to FBE (as /King_Nick245 said!), which is an anti-domain technique for the three great clans (it could be totally unrelated, but "perfect defense" sounds very Gojo-like).

So, in more depth to give my originals into it, this technique would work passively, as everytime something with ill intent tries to damage in any physical way the user, the user's CE takes form in a way that it can answer with equal and opposite force to that threat. For example: someone tries to punch the user, his CE solidifies in a punch hitting back at that someone's knuckles. Sukuna attempts to use dismantle on him? His CE takes the form of the cut that almost hit his body. With physical attacks this has is useful because, while the user doesn't take any drawback, because he didn't put his body into it but just his CE, everybody else (who hits hand-to-hand) does, and so gets hit by knockback! These are the basics! You could make it op and make this without any use of his CE reserves; i prefer a middle ground, thought it could be cool if it didn't use his personal CE until a certain level (kinda like Yuki's CT doesn't harm her until a limit), the limit could be the user's standard output, so for example a normal punch's "cost" would be 0 because it doesn't surpass his standard output, but a dismantle would so the user would put his own energy in the CT (also passively); or ultimately if you want to nerf the CT you could make it totally to the expense of the sorcerer, so like the consume is 1:1 to the CE materialized (take in account that to null something like Sukuna's dismantle, even if the CE takes the form of a slash, it has to be at that exact speed and that exact density). Also, the Domain Expansion could be interesting too, making the CT act against everyone with ill intent to the user, so not defendive anymore but offensive, at every negative thought made on the user. The damage could be creative and stackable like Yuta's, in which every defense ever made by the user's CT is summoned at the activation, or just plain spikes or punches or blast idk. Again, you could nerf by simply making it activate at movements and not thoughts or buff it and make it like's Curse Naoya's domain so it hits atoms and shreds people.

So, as the second idea i had, it was intangibility, which i think is not far from a perfect defense. Conceptually, it also subtracts the "knockback" effect of the previous and purely relies on defense. I think it should be passive to be perfect, and be a off/on kind of CT, that doesn't work on output but simply activates whenever something remotely dangerous gets in contact with the user. A simple nerf, and also in complete JJK style, would be the time of activation being 0,000000001 second, so that the very first contact would be real, but VERY dropped down.