Internally reinforce the chainstay with a piece of dowel, notch it in the desired area and then lay glass and/or carbon cloth over the filed out area and wrap the lug.
I usually lay a strip of glass cloth from the downtube across the bb and onto the notched chainstay and then a strip of woven carbon tape and a strip of unidirectional carbon.
Three layers plus the tow wrapping more than compensates for the filed out area.
You will have to get a good idea for how much thickness of material you are accumulating to make sure you still have the clearance needed.
The other choice is to select a piece of bamboo with a natural kink/curve in the right spot.