The cervelliere's chain mail skirt, called a camail or aventail, was not attached directly to the helm. The mail hung from a leather strip, and the leather strip was attached to posts, or vervelles, along the rim of the helm.
This detail from the funeral brass of Thomas Cobham, dated 1367, shows a thong running through vervelles on the edge of his bascinet helm.
Vervelles are not available commercially. They require some creativity. These are made of the posts from Chippendale drawer pulls, with the half-holes drilled through. I bought the parts at D. Lawless Hardware.