Just done that, and had a look. I couldn’t understand at first why the centre was black, until I looked more closely. You have FAR too many segments in your curves there.
It’s only when I zoom in that I can see the individual lines. And there are a lot of holes, because some edges are too short for SU to recognise.
Let me try drawing it again…
I draw the ‘clamshell’ first, as I showed above, but using your dimensions - 26" outside radius, and 20" inside (your inside points are 19 65/64" which makes me think you intended 20")
Make life easier for yourself and centre things at the origin.
[Pause] SU is misbehaving - the Rotate tool isn’t working as it should. Will have to restart my machine.
Another pause - for thought.You have drawn 13 sections in your clamshell, which gets an opening in the centre at the top. The model you referred to in the 3D Warehouse has a point in the centre, with a much easier-to-use 15° rotation between grooves, rather than the awkward ~13.845…° needed for 13 sections.
I’ll draw it with 15° rotation. It makes alignment with the centre boss much easier.
Drew the shell segments as before, 12 of them, with 12 segment large (20" R) arcs, and 8-segment semicircle ends. Run Curviloft/Skin countours to create the skin of the segment
Centre boss - draw a circle with 48 sides, push pull to the right thickness - protruding from the clamshell by 1" copying your model.
Line up the profile, and rotate it so it aligns with the MIDDLE of an edge of the circle. That way, it starts at right angles to the follow me path, and produces a clean round centre, not the U shape you had.
Run FollowMe to create a full circular boss, here shown with hidden lines showing.
To cut the boss in half, draw a rectangle through its horizontal diameter within the context of the component.
Intersect that face with with context, check that the front outside face splits in half (mine didn’t so I drew to lines to get it to split).
Then change to Camera/Parallel projection and View/Standard Views/Front.
Draw a R-L selection box to select the lower half, then delete it. Tidy up any stray edges, including the front of the rectangle.
Exit the editing context, and R-click Change axes to place the origin of the component at the centre of the boss.
Turn off View/Hidden geometry and you get the basics of the top of the niche.
Add the front rim. Draw a 26" R circle, select it, then Intersect Faces/With model to get this:
Erase all the unselected faces in this image, reverse the face shown selected, and push pull it (say) 2" to create the rim.
Sorry, forgot to upload the SKP file.
Greek Arch SU v2017.skp (2.3 MB)