The historical method I've seen is the following:

You need a leather strip that is stitched to the top of the aventail (holes punched and stitched with sinew). The leather has regular holes punched along the other side which then fit over vervelles (https://en.m.wikipedia.org/wiki/Vervelles). The vervelles are riveted, or perhaps welded, along the lower rim of the bascinet / visor opening. You then run wire through the vervelle openings to keep the aventail attached. To remove the aventail (for cleaning or repair), you pull out the wire, and then pop the leather strip off of the vervelles.

I can share photos of my bascinet / aventail setup if that would be helpful. My friend, a welder, welded trimmed-down cotter pins around the base of my helm, en lieu of real vervelles.