This portrait showing Capt Ranald McKinnon of the 84th Regt has a clear depiction of the loop at the corner of the plaid that would have been used to attach it to a button or the epaulette. There is a similar one shown in the portrait of Lord Loudoun.

Click image for larger version.  Name: 84th Regt - Capt Ranald McKinnon.jpg  Views: 27  Size: 100.7 KB  ID: 28892