I was asked to make a quilt for an occasion that involved a lot of children. From experience, I realized that non-sewers/quilters, do not 'get' the idea of a seam allowance. Soooo...
I cut the squares of plain fabric and added a 1.5 inch border around each. I chose border colors to fit the occasion and several different prints in each color. As the borders/sashing, were already on, so no one could go outside into the seam allowance. People picked the color they wished to write on. The cardboard is a good idea, but we had a table and several chairs, plus a box of fabric markers.
After they were signed and 'messaged', I laid them out according to color and sewed them together -- voila, quilt done !