I used tag board to determine the size and shape I wanted, then I did some experiments to see how much the Fosshape (a kind of felted stuff that compacts and hardens when steam ironed) shrunk. Looks like it shrinks about a quarter inch per starting inch. Then I cut out the amount of Fosshape I thought I would need (plus a little) and ironed it over a humidifier tank. lol. But, hey, it worked, didn't it?
Then I used my tag board pattern to both determine the Fossshape's final shape (and cut it out that way) and to mark the shape on the fabric I planned to cover the Fosshape with. I used a black pen to draw my best impression of the pattern the guard should have and cut the fabric out with a ~1 inch allowance.
Then I used basting spray on the fabric, smoothed it over the Fossshape and used hot glue to attach the allowance to the back of the Fosshape.
Don't laugh, it worked!