So instead of cutting it straight off you cut along the frame?
Exactly. If you cut straight across, your undertail will have no rear support.
It will flap up and down at the area where the rear cowl covers it.
The previous owner did that and I had to drill a hole through both and use a body clip to prevent it.
You can see where I cut the fender along the frame rails towards the battery box area (it's an SP2).
This leaves the outer portions of the fender mounted to the frame rails.
Not only does it retain the relay mount tabs, it also allows additional attachment points for the undertail at the front and rear, in addition to the body clip point where the rear cowl attaches.
I used some scrap from the fender to fashion mounting tabs for the rear portion of the undertail, and used a couple of aluminum windscreen mount screws and nylocks to attache the undertail to the tabs.
I chose a mold line near where the tool kit went. If I had it to do again, I may have went a bit more forward with my cut up to the tool kit mount.
I use the took kit mount to hold my power commander.
Also, the undertail had some minor interference with the fender portion where the ECU is mounted (on SP2). I had to use a demel tool to slightly radius the undertail so it would wrap around that part for a better fit.