We also need to keep in mind that - given the widths of looms - the plaid consisted of two pieces of material sewn together. It would have been (as it now is) very easy to stitch the two together such that the stitching itself accomodated a belt or drawstring of sorts that when tightened would automatically create nice even folds all around, easily flattened by another belt into box pleats. I haven't tried this on a full scale, but have experimented with a small pieces of cloth and it works perfectly.