Attachment flat/flush over top of the electric fixtures box would need an electrical box coverer first.
On my idea I meant leave the fixture in place and build an box frame around with the backdrop an the boxes face, not refering to structure or an electrical box code; thats a given. Just an "artistic lamp shade". It can hang like a picture frame or be layout attached and pushed against the wall. Even screwed to the wall. Houshold wires splices the runs must be anchored firm and contained to stop shorts & spred of flame should a short ever occur.
Properly ventilated shade so it doesnt become an easybake oven 
L.E.D bulbs can solve that now too. Ive seen plenty of similar concept built custom fixtures in businesses and theater everywhere.
