Am I too late??
If not, here's my 2 cents.
1v1 or 2v2. Draft pick or blind pick. I've been in ones that don't use draft but still require you to ban, it can get a bit confusing and ends up taking more time.
Since it's Proving Grounds, I'd probably suggest a full game. They don't take too long usually and you can then double check who won without the need for screenshots or a referee.
Starting at about 3-4pm on the first day going for up to 5 hours.
Depending on the turnout, if all of the matches aren't finished by then it moves on to a second day starting earlier than the first(suggested 12pm). Continue for 7 hours then repeat until finished.
I would suggest starting this on a Friday or Saturday so at the very least there are two days for it.