There are many ways to make the connection. Here is one which takes advantage of the tapped hole in the aluminium footbrace rail ( the thread is standard 1/4-20 if you need to ask for screws in a hardware store).

1. First, take the end of the nylon webbing, fold it over and sandwich a 5/8" (~16mm) stainless washer in the fold. Hold it together with a spring clamp as in the photo. Make sure the 1/4" screw can pass through the washer first :)

I was popping veins in my face to rip it out but to no avail. It is plenty strong for this application. Trim the screw so that it is flush with the underside of the rail. The footbrace can now freely slide in the track and the low profile webbing attachment is out of the way so there is no snagging on the corners of the plastic track.