If you are considering a one-off event, I’d recommend a team tournament. It has the feel of a mini-circuit.
The way we did it in Michigan, is a few months before the event, I asked the Michigan T.O.'s (basically the T.O.'s that run the store championships) if they were interested in putting together a team for a team tournament. We had 6 stores that were interested in sending a team of four players. Whoever was interested stepped up to be the captain/organizer of their store’s team. They had the freedom to choose their team however they wanted. For my team, I had a GNK tournament for whoever was interested in being on the GYGO team and the top 4 made it. The farthest out team was 2.5 hours, but everyone else was within an hour. California can probably do 2-4 of these.
I talked to one of the stores about setting aside a 6-hour block of time for 20-40-person event. I got in touch with our local GLC rep about getting some ANRPC prizes. Five dollars per player was enough to cover 2 GNK, ANRPC dice for every player, as well as custom playmats for each player of the top team. But, the biggest prize was playing a whole event as a team where each faction is represented equally and can table-talk.