Why not try your hand at a wholecloth? You could get it all marked out by then quite easily or you can purchase one of the pre-printed wholecloths found on line. Our member Holice has some and you can also find some at the Stencil Company:
http://quiltingstencils.com/preprintedwholecloth.aspx
they have mini ones and bed sizes here.
As the demonstration is on your handquilting technique, I would think it most appropriate to showcase your handquilting skills instead of your piecing skills and nothing will do that better than a wholecloth. You can start working on the wholecloth prior to the demonstration so a large portion of it will be completed for showing participants as well as being able to demonstrate the technique itself in the unfinished portion.