Perhaps you could do a non-judged show that had an interesting theme, such as, "the deep blue sea," or, "city lights." or "just cats." It my take their focus off of the actual, construction details and put more on the creative process in meeting the needs of the theme.
Also, if you're doing a judged show, little prizes aren't much of an incentive, but big ones are. A free weekend in a favorite city, or country B&B, or maybe a slew of travel points to spend how they wish, would nudge them into entering. Sponsors may be able to help you out with that one.
~ Cindy