Becca,
If we have enough prizes it would be nice to do two prizes per week, one for the best one, and one prize for a random drawing of all entries. You will need to arrange for judges. I would suggest you keep the judges confidential, and state that their decision is final. Pick anybody you want for the judge/judges, someone you could show the pictures to and get a quick decision.
Do you want to have them enter by E-Mail, or by posting on the forum? Each way has it's pluses and minuses.
For the second week, does it need to be ALL wood? Or will you accept one that is mostly wood with some other substance? How about stabilized blanks? How about wood/resin composits, such as the worthless wood? It might be easier to say "all wood, no resin allowed other than for a finish. But you cut out a lot of beautiful stabilized blanks that way. I'm not suggesting one way or the other, but you should have an exact specification because people will question you otherwise.
For week four, you could say "material or materials". That way more than one material could be used, but they would all have to be present in both the pen and the stand.
Scott.