This is a simple hook designed to fit 1515 aluminum extrusion.  There's confusion about that designation sometimes, so to clarify, that is 1.5" by 1.5", NOT 15cm by 15cm, extrusion.  They're good for moderate weight at most; they are not designed to hold much load.  A trash bag, some clothes, or a backpack is about all I'd recommend.  The part has a bump-out piece which slots into the track to keep the hook from turning.

You will need some method of attaching these to the extrusion: either a knob and an insert, or a hammer bolt and a nut or some similar arrangement.  In the photos, you'll see a 5/16-18 hammer bolt and a printed knob with tee nut insert.  The hole in the model accommodates a 5/16" screw, but you certainly don't need anything like that size for the loads this can carry.  

Use filament of your choice.  I use PETG, but there's no reason this won't work with just about anything.  I recommend increasing infill to 40-50% and using four perimeters.  It will print without supports with the hook side flat on the platen.