I say again- it's a T55Solution.!
I did the same myself the other day, I used a T50 Torx which rounded the corners, I then got a bigger T55 & hammered it fully home, result no slippage..
At first it appeared the T55 was too big, but it's a really tight fit & needed hammering home.
The nuts are heavily coated with loc tite, making it hard to remove.
I didn't but poss should have softened with WD40..
Please give this a try before drilling..
Shakey's law - "if the Torx bit fits, try the next size up just to be certain. If THAT fits, refer to Shakey's law"