A box isn't a bad idea...might be a tad expensive if you do it on a regular basis though. We play chaos EDH every week down here in Kansas and we usually get around 8 people, sometimes more. We usually have two tables playing and the top 2 players from each table move on to a final table(3 points for win at first table, 2 for second place at first table). The same point system applies to the second table and the person with the most points at the end of day, usually takes home an original dual land(sometimes other older money cards like phyrexian dreadnoughts and goyfs and whatnot--sometimes a few Bobs or other extended things). I think that sort of prize is good, because it allows players to further enhance their edh decks with better fixing or good cards and it also helps players who have maybe only played standard the chance to get a jump into extended or legacy, since the duals and some of the money cards are the main reason many players avoid those awesome formats.
I don't know about other people, but I love going to EDH each week because the format is fun and dynamic. It also helps if there is incentive and the older cards we give away down here bring in a lot of people, don't cost as much as say a box would, and have started a lot of people playing legacy(which is my personal favorite constructed format).
I hope some of these suggestions help. More people need to play the greatness that is EDH.