I'll show you how to add thickness to surfaces. Select at least one point
of the the last row, do Shift-R one or twice, Extrude to the left, Scale
down, apply Weight:9 or 10, Extrude again, to the right now, and Scale down.
When you need to close holes or areas do the following. Convert the Surface
to Mesh with Alt-C, carefully select the last row of vertices, follow the
guidelines to select the entire row, then press Shift-H to hide the non-selected
verts ( see image above center right), now, you can easily select 3 or 4
vertices to make faces, with F key.