I started down the path of building a simple version of this at one point. One thing I discovered is the necessity of marking some time as busy within the window for an event. If I have a 2 hour block free for a 1 hour lunch, someone could still schedule a meeting which makes my lunch impossible, like a hour meeting starting 30 minutes in. That would leave two 30 minute chunks on either side, and I can no longer have my 1 hour lunch.

The solution is to preemptively block key times which guarantee you can still schedule your flexible task. In this case, you need to block 30 minutes (or whatever your smallest schedulable unit of time is) in the middle of the 2 hour window, either right before or right after the midpoint. This will leave the maximal amount of schedulable time while still guaranteeing no single event can wreck your lunch.

Just an observation in case it's useful to the founders here.