I have been making padded aventails for a few years for my husband to wear when fighting heavy combat. This is some of what I have learned in further researching them and experimenting with the design.
There are not a lot of images of padded aventails, but there are a couple of very detailed effigy sculptures. The effigy of Philip the Bold shows the joining between the aventail and the helmet liner and the shape of the channels on each and on the effigy of Sir Walter Von Hoenklingen the aventail looks like it might be attached to the outside of the helmet rather than the inside. The aventails in the fresco from St. Stephen’s Church, Milan, Italy do not give any indication as to whether they are padded or mail, but drape with a similar stiffness as the other images and personal experience. Additionally, as on the effigy of Philip the Bold many of the mail aventails could have also had a padded aventail under them. This would provide different types of protection, with the mail protecting against cuts and the padded aventail protecting against concussive hits and also protect the wearer from the mail.