I've made a couple for customers. I just use two metres of double-width cloth and fringe the two short ends. If you look at most of the men in that picture you'll note their plaids look to be the same fabric as their kilts. This produces a much less cumbersome plaid to wear than using a picnic blanket.

The man third from the left has his plaid worn across his chest and then draped over his shoulder. I would imagine it is easier to keep on that way.