The clip can be slotted into 2020 extrusion from the side, and then rotated to lock them in place. They can be quite stiff to get in, I used an adjustable wrench. I've included a 3mm hex wrench socket that may also work to turn them into position, but it wasn't as reliable for me.
The parallel/perpendicular naming refers to the direction of the velcro after the clip has been rotated into place. I use perpendicular for most of my cable management.
The clip's diagnol dimension is less than 20mm, so you can use these even on 2020 rail that has been mounted against a solid surface, like the underside of a desk.
To print, I used supports on the tab that goes into the 2020 rail, but the rest of the print doesn't need support. Print so that the flat sides of the tab are vertical, for best dimensional accuracy.
You can use the publicly accessible Onshape document to make further modifications:
https://cad.onshape.com/documents/9970a16af131d5cf17f47300/w/c527dd99e32f289a03efa2c8/e/c34fa65a98f4adb148519f6f?configuration=List_1qMHnqz3F5zjWX%3DDefault&renderMode=0&uiState=655009457f53951907cb0cfa
The author remixed this model.