Idea #1:
I'm thinking of those table centerpieces that stand up with foil-like spikes (like a firework) and at the end of each, you could have a star with a detail.
I'm sure they ave them in red, white and blue. Here's O. from party City:
http://www.partycity.com/product/star+cascade+centerpiece...
Idea #2:
What about an Uncle Sam hat with a detail on each stripe?:
http://www.partycity.com/product/patriotic+glitter+top+ha...
Idea #3:
Make the poster board into a flag and put O. detail on each stripe!
Idea #4:
Make the poster board into a "Wanted" poster. Place a picture of Uncle Sam in the picture box, burn around the edges to make it look "old" and bullet point the qualifications and compensation with flags or a symbol for each (dollar sign for salary, etc.
Good luck. Sounds like a super cool, fun project!
Personally, I vote for something 3D rather than a poster board (#1 or #2 above) it will just make for a more interesting display. And interactive and walk-around is always a bonus!