Here is an example where we don't even need to use the Solidify modifier. This piece has a front and sides. All we need to do is give it a back.
Switch to edge select mode and select one of the outer edges.
Go to Select > Edge Loops to select the rest of the ring.
Select > Edge Loops
Press ALT+F to fill in the back with polygons.
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Insert a new line below
Insert a new line above
Indent current line
Unindent current line
Delete current line
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