After all, if the enclosure were, say, two molded rectangles of glass, there is nothing to limit the shape of the glass pieces, assuming that they are molded. On of the discussions on alt.solar.thermal from a very bright engineer named Anco was that glass Fresnel lenses were the way to go for point focus concentrator pv systems, too. As I recall, some group in China was supposedly saying that they could make Fresnel glass lenses. The problem with making Fresnel lenses is that it takes sharp corners to make the Fresnel lens efficient, but casting sharp corners into a brittle material like glass is asking for trouble. Glass breaks, you know.
Sharp edges cause stress concentration risers, complicating the problem.
A molded glass, water filled secondary concentrator avoids the stress risers, and eliminates the problem of casting sharp Fresnel features in a viscous glass material. The front spherical surface is usually very curved, and the back surface is almost planar. I have an old patent that is now expired where I used line focus elements to increase the concentration of a primary linear focusing element. I used cylinder lenses for my first test prototypes back around 1970 when I was but a teenage freshman in physics. I found the effect of shortening of the focal distance with increasing off-axis angles.