2 separate trim tabs per side measuring 30mm wide x 50mm long (60mm total width per side).
Independantly adjustable on one mounting bracket
Use the 2 the existing mounting holes then drill 1x 4mm hole for the 3rd bracket hole (closest to centre). Do not attempt to drill a hole for the 4th mounting bolt because it lines up with the rails (unless you intend to use a self tapping screw)
3 bolts is more than enough to mount these tabs anyhow