Guy rope tensioner. There are already quite a few designs available on thingiverse.
I quickly designed this one for thin braided nylon rope (provided with my cheap gazebo).
Diameter of the holes is 3.5mm and it fits approx 3mm thick line. It holds tension well with single figure-of-eight knot at the end of the guy line.
3.5mm hole diameter
Print in ABS or Nylon
Fits 3mm rope