I created a cap to fit into the open side of the frame for the Halloween silhoutte scenes by 3DPrintBunny listed below
https://www.printables.com/model/582917-skeleton-multilayer-silhouette
https://www.printables.com/model/582918-halloween
Since they use a license that does not allow remixes, I redrew the geometry in onshape rather than just using a cut and a negative shape modifier from the original frame stl.
The author marked this model as their own original creation.