Summary

I like the design by Mattasmack and have tweaked it a little bit.
I like things hidden so moved the bracket in the centre so added a transition piece (orange) to achieve this. The bracket (green)and the shade (red) now become symmetrical and you get another degree of freedom as well. The swing arm can also be made smaller, use the "swing arm short" file. A screw attaches the transition piece to the bracket. A bolt and wing nut can be used to tie the transition block and swing arm together. The "wall sconce centered" file is for illustration of the assembly only. Also look at http://www.thingiverse.com/thing:488720 it is an improvement IMHO.