I think I would have the fabric washed, blocks made, and have pigma pens next to a stack of blocks. Instead of a register or next to the register and catch people as they go in. You may have to post a "guard" so everyone knows what to do. Catch them before they hit the bar! If there is a young person they would probably be honored to be the guard! If there will be children there, I think I would have plain light blocks that maybe they could draw a picture on with crayons. Make sure you have any seam lines drawn so no message gets caught it a seam.