I have a large Thule Alpine something-hundred roofbox. I moved to aerobars with t-track fitting. To make this as secure as possible I fabricated some large 2 or 3mm alloy spreader plates and rivetted and jbweleded them inside the box covering the hole zone. I then re-drilled the holes. The plates are maybe 100mm wide, 300mm long.
I reckon the t-track bolt to the box is a stronger interface than the t-track head in the track on the roof bar now.
Generally my roofbox is rammed full with skis and poles, then boots and helmets, then soft stuff. Once the lid is closed, nothing is moving in there.
I trust the 3 point locking mechanism. I notice all the Dutch and Belgians have an extra orange strap flapping about around the outside of the box. I award points for outstanding vibro-strap.