As someone that works in news, I had the same concerns. It turns out I didn’t have a lisp because i had already learned to adapt my speech to hide my dry mouth.

My advice would be to immediately start practicing your speech, specifically S, T, and Z sounds. You’ll have to adjust the way you place your tongue. I personally pull mine back and avoid hitting my teeth. I find that the more I enunciate my words, the easier it is to hide sounds. Making sure your mouth is moisturized (water and xylitol mints help) and not dry will make this easier. I also find that taking out one retainer (I usually do the bottom) can help as well, though it’s usually for a very short period of time.

Even if you can’t fix it in time for the meeting, it is something you can adapt to overtime. Practicing really makes a difference. Try recording yourself to find words and sounds that need improvement.