external case and overheating

My wife has a new macbook and is looking at cases and sleeves for it. My concern is that with the fanless design, a typical clip on plastic case will raise the computer's temperature and at the very least cause increased throttling. I initiated a chat on Apple support but I suspect I was not talking to a real person as I got a generic reply saying that all mac laptops have an internal fan so a clip on case with vents in the right places will not cause cooling problems....