I used Plaid Stiffy Fabric Stiffener! And it does what it says ~ its a water based formula and non-toxic.
I also tried spray starch but it wasn't as effective. Stiffy really made the fabric almost like paper. You can dilute it with water but I just used it straight and I was happy with the result. The stiffer the better! And it helps prevent fraying of the edges of the fabric.