I would not still recommend setup from the first post in this thread. I now have a much more elaborate vacuum board/strobe setup.
For quick pictures, I would now just recommend you take your poster outside in the shade somewhere. Bright (but not direct) natural light always seems to give the best results. Set out a piece of foam core first so you have a clean, flat surface and maybe add in a color chart if you want to tweak later. The posters need to be flat or weighted in the corners though.