Capturing an image on their cell phone picturing what they have not yet achieved might allow them to try your hands-on exercise later, where they feel it's okay to "fail."
When I teach segments that require math and also those segments that requires specific motor skills like art or musical performance, I find it is always hard to get student started. Adult learners are especially skeptical that their early attempts might be judged as being clumsy or awkward.
Really, really small steps or parts of the desired structure, that are easily mastered, will build confidence for them to draw the whole structure in short order.
Good luck! I'm sure you will be amazed at their creativity once you unleash them from their earlier programmed fear of failure.