Custom

You can define your own custom Mesh. To do this, you need to specify a set of vertices in the Custom Geometry field: 

// Vertical Plane
0, 0, 0
1, 0, 0
0, 1, 0
1, 1, 0

The order to specify vertices is to follow a backwards Z pattern for each face:

Order of vertices per face on a custom shape

Repeat for each adjacent face until you have created the shape you want.