You could split the contest up into three stages...
(1) Registration. Anyone who wants to compete has to send in a brief explanation of 1-3 skin concepts. That explanation must have:
- Champion
- Working Title
- A brief, ~1 sentence description
- IF you choose to allow concepts that have already been made public, a link to the first time you made it public (it has to be somewhere that would have a timestamp).
(2) Disclosure. After registration ends, post a list of all the attempted registrations. All duplicates are marked as disqualified (but still listed) unless one person has posted a link proving they were first. For the next 48 hours or so, anyone who has seen any of these concepts before can post a link to a timestamped thread or online portfolio proving prior art, in which case these concepts are also disqualified.
(3) Presentation. Now the contest starts. People pick one or more of their concepts that were not rejected and start writing up proper descriptions and/or drawing artwork to convince the judges that their concept deserves to win. You could technically start earlier, but it's not until stage 3 starts that you know your concept isn't going to be disqualified for duplication or prior art.