May I suggest that you make a separation between the cut-off date and the judging date?
I say that because getting a few short stories might not give you much time to read through them and stuff.. especially if the authors wait until the last minute to finish going over them, and then send them in at the last minute. (just ask Tlither about the E-zine deadlines!

)
Also, I don't want to start a riot here, and ask that no one start going nuts with this next suggestion.. but perhaps you should narrow down entries and then have folks vote on them in a poll?
If nothing else, it puts more "eyes" on the contest, so anyone taking a short story off of HBO, or graphic off of Deviant Art, and pretending it's their own can be busted if they're "in the running". Also, I certainly don't envy your position of having to judge all these things and determining if a short film is better or worse than a story, and/or artwork/photo, and this way it's not just you.
Also, if this is repayment to the 405th folks that helped you out, you might want to consider a "joined by" date requirement, to avoid a flood of new folks when word leaks out.
If I can help you out somehow, let me know, I won't participate even though I'd love to have one of those for myself and have always wanted a legitimate reason to do a Halo short story, lol.