I often connect people that need designers or translators with actual designers or translators.  My advise is that you will get a better design and better relationships with better designers if you take a look at someone's portfolio and hiring that designer based on their previous work.  The reason is, it is not fair to have a multiple designers do work for free hoping that you might hire them.  Most of the designers that do spec work are not good designers.

Also, with the nature of the designing business, designers put risk to their livelihoods posting outlines of a work.  It could lead other people to make more designs based on that work.  The person that did the most work in coming up with the novel design does not get the job, and the unimaginative designer that tweaked the initial design does.

Unfortunately, all my designers are booked up, but my advise is to ask to look at portfolios of designers and choose a few you like.  Based on those do some interviews and give them some ideas of the work you want.  From that hire the designer and put the money in escrow until completion of the work to your set contract.  I guarantee you will get a lot more interest from designers and a better piece of art will be produced.