Maybe something like this ;) (just some speedy tips so make it with more precision :) And there are always some possible methods !
Of course you can fillet the edges :)
http://www.divshare.com/download/2889362-cef
Unzip and launch in you favorite navigator the file named "ring.htm"
For the top part just make some Boolean operations
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Pilou
Is beautiful that please without concept!
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