This is a simple bracket that fits on most standard roof racks. It has 3 holes that can be used for whatever you want (the outside holes are to bolt it to each other, and I used the middle hole to attach an LED camping light). But could also be used to attach bungee cords or whatever.

The center hole is 10mm in diameter, outside holes are 8mm diameter.

Total width of the bracket is 111mm, with 77mm inside distance between bars.  

I used PETG which has held up through summers and winters for other parts.