How to create a shell niche

A few nights ago I was having dinner in a nice Italian restaurant with a neo-classic decoration. A few walls had shell niches that exhibited wonderful sculptures. These niches were exedras - a semicircular recess inside the wall.

The word Niche derives from the Latin nidus or nest. The Italian nicchio for a sea-shell may also be involved, as the traditional decoration for the top of a niche is a scallop shell. I recall a nice piece of art of Filippo Lippi - Madonna - where the trompe-l'oeil niche frames her.
I took a picture with my phone with a very bad light just to remember the image.

I thought it could be a very good example to apply the Extrude tools plugin developed by TGI. After registering you can download them from http://forums.sketchucation.com/viewtopic.php?p=217663#p217663 .
You will also need another plugin called Weld from Smustard that can be downloaded from http://www.smustard.com/script/Weld . All these plugins are free.
Once you have downloaded both tools follow the instructions how to install them.
These tools are a must-to-have for interior designers.
After analyzing the geometry I decided that the option to use was "Extrude Edges by Rails." I had organic forms with double curvature, the front arcs and two curved paths converging at the rear of the exedra.
I followed this procedure:
1. I drew a 12 segments half-circle in the red-green plane. I drew another half-circle in the red-blue plane with the same properties that would be the opening of the niche. I made a third half-circle with an offset at 2” to get a face to work on and to use it in a later step to finish the overall niche.

I exploded the “opening” arc to get separate segments. I divided the second segment into 6 and drew a big arc and two small on each side. I worked with the second segment of the arc because it was above the bottom part of my drawing and was easier for me to work with.

I drew a line from the center point to the base arc to divide it intn two parts and work only with the curve that I was going to use as rail. I rotate + copy 15 degrees and 30 degrees, and drew a short arc between the two rails to define the shell focal point.

Next, I erased the construction lines and I joined with Weld the three small arcs of the wedge.

I selected Extrude Edges by Rails from the Extrude toolbar and selected Arc 1 as profile curve, Arc 2 and 3 as rail curves, and Arc 4 as melding-profile.

I reversed faces, erased coplanar edges, smooth edges and erased original curves.

Once the group was created I rotated + copy 11 times every 15 degrees and I made a group. With Intersect Faces > With Selection I divided the front face into two separate ones so I could erase the face covering the niche space to reve`l the inside of the niche.

Finally I drew a profile in the top edge of the cylinder and I used Follow-Me to create a molding to separate the cylinder from the exedra. ( I could also have used Extrude by Lathe)
To create a fast rendering I applied gold color inside the niche. I imported an alpha-transparent background image “as image” and applied it on a vertical face. I created an Always Face Camera component. I edited the component and erase the vertical face keeping only the image. I placed the Rodin sculpture inside the niche. I turned the edges and profiles off,

I used Pull/Push to create the lower part of the niche and Line to complete missing faces.