Greenhouse T-slot nut / eyelet

Two types of T-nut to slot into greenhouse aluminum extrusions. 

A solid ring to give the strongest attachment 

and a split ring where you can push a string into it and it will clip inside and hold it, 
but it will not come back out.

Just slot into your rail and turn 90deg, it can be a bit stiff at first so maybe use plyers or i put a nail through the loop and use that to turn it. Once in its rock solid and load bearing. 

The T-Nut from the original model was used as it gave a great firm grip on the rail.
But the body of it used more filament than needed and the shape wasn't necessarily the strongest. So These too new versions are designed to be stronger for less filament. 
which is important if you print 20 of them to deck out your greenhouse. 
i've used them to fix string lines in by greenhouse to support heavy tomato plants and stop the bamboo poles from tipping over from the weight.