A fly plaid is (usually) a square piece of tartan, (sometimes) fringed on all 4 sides, (sometimes) on 2 opposite sides, (sometimes) not fringed but purled on 2 or 4 sides. I'm using parentheses because not everyone agrees on fly plaids. The corner of the square is (usually) sewn into pleats, through which a brooch pins it to the left side of your coat, beside the lapel. A fly plaid should be a minimum of 40" square, which is why I said you'd have to sew two pieces together to make it wide enough.
Some people like fly plaids; some hate them. If you search this forum or Google, you'll see pictures which explain fly plaids better then I have here. If you ask opinions of them, the thread will run on for years.
I hope this helps.