I’d be wary of creating a stress riser. Creating a rigid tail without connection to the primary strength of the frame (the tunnel) will focus the new stressors at your connection point. Which is just pressed 1mm steel somewhere through the back of the tower, across the top of the wheel well. Could fold your tail... Or tear it off.
The strength of the frame ends at the engine mount. Fork tunnel at the front... frame tunnel to engine mount. The leg shield and floor add significant strength as well by completing the tunnel making it an arched tube - but mostly just the middle bit across the tunnel. The rest is just creature comforts.
What about above the engine mounts? Could you find a mounting point there - perhaps above the kick start? You’d want to connect both sides of the frame - an engine mount bolt with solid tube inside might work. Or weld.
Just my thoughts... minimal experience as you know.