20 mins prep is indeed super short, is not the conventional written case, that are indeed becoming very fashionable nowadays as a way to interview!
Remember that the skillset tested is the same than in the "usual" cases, hence, all the practice you may have done totally plays in your favor.
One important point to add is the need to be very 80-20, structured and to the point, since the prep time is very short, so we need discipline with the analysis to have enaugh time to prep the communication strategy. In the case of 20 mins even more, you won´t be able to do more than 2-3 slides.
There are many many entries in thsi same Q&A regarding written cases, hence, I would recommend you to look with the keywords "written case"
Hope it helps!