I saw a suggestion on another site recently where a piece of plexiglass, about 18in square was laid on top of the quilt. (cover the edges of the plexiglass first with tape, so there is no danger of writing on your quilt because you can't see the edge of your pglass).
Centre the plexiglass over your block, and useing a felt tip pen that will wipe off, try out a quilt design. If you don't like it, wipe and try another one.