I had an offset cup set and I had already ground for another issue so I prepared that for this task, but. I ended up removing the retainer spring from the 3X gun and used a mushroom set WHITHOUT a rubber guard so I could get as close to the flap bracket. This worked okay, not ideal but good enough. I would want just a bit better or easier method if this were on top.
RV-10 tail kit 50%
(When do I get to measure something?)
"Flying north of the Arctic Circle and back - several thousand dollars,
Doing it in an RV you built yourself - PRICELESS!"