Harvard's Natural Sciences Lecture Demonstrations group films short demos as a resource for teachers. This 2011 video shows four centrally mounted brass plates, two circular, one square and one stadium-shaped, driven with a violin bow while sand shows the nodes.
The detail worth taking away is in the description: pressing a fingernail against the edge imposes a boundary condition, and so does where you bow. That is how a skilled demonstrator selects a mode without a signal generator, and it is why the same plate can give different patterns in different hands. The group's website, linked from the video, has the plate dimensions.
The team apologise in the description for continuity errors. Ignore them; the plates are good and the variety of shapes is the point.