Gentlemen there are many references to the dress of the highlanders of Scotland in the 16th Century, and many of them refer to their cloaks of various colours but with no mention of a belt to drawn it around them. Those same works refer to the yellow long style shirt/leine being worn by those highlanders with the plaids. The first mention of the plaid wrapped around them with a belt to secure same was, as has been pointed out, 1594.
The up to 50 year before that time is little but conjecture as there are descriptions covering that time frame but none mention a belt , of any kind being used to secure the plaid around the waist (1548, 1573, 1579, 1581, Even 1594 in other description outside that of description of the Hebrideans that went to O'Donnells aid,which is the first record of the belt being used)