This is a simple way to quickly adjust a continous ridgeline or the attachment points of a tarp. Whether on trees or ground anchors.

Simply attach the fixed end to the eye. The end to be tensioned is first passed through the hook, tightened and then pulled over the antenna through the gap. 
For additional hold, the process can be repeated crosswise.

The original design comes from Dutchware. On their site you can find more information about how to use it.
I modeled it from this template.

Printing Notes:

• Resolution: 0.2
• Filling: -
• Walls: 3
• Bottom/Top: 2mm (so that it's massiv)
• Material: recommended PETG or ABS
• Supports: -

I recommend PETG or ABS, PLA could break or deform due to the sun but should work as well if it's not too hot.