Remove the wire-lock pins and slide the basket to extended position.
You'll note that the further back you slide the basket, the more it wants to pull down. You are looking for the magic spot - long enough for your envisioned load, yet still balanced with the runners. When you find it, drill through the inner runner rail at the point of the existing hole on the exterior runner rail.
Slide the wire-lock pins in and give it a shake.
"it's a bit shakier, but it ain't goin' nowheres," you should be saying.
Feel free to refer to the extended basket as "X-wing position."