Are you beeing stubborn or is your wallet crying at the thought of buying ANY fabric? IF you are being stubborn and use the "I'll be damned if this fabric is going to get the better of me" approach, then go ahead and fight it to the bitter end. :D
There is some basic solid color cotton available for a few bucks a yard and it would make your life so much easier. It may not be the best quality either, but at least it would not stretch or distort.
That said, iron-on interfacing is available in various thicknesses and the thicker it is, the stiffer the end result will be. Your best bet is to take a piece of your fabric to the fabric store and ask the nice lady what to back it with. If your fabric is polyester, you want something that bonds at a low heat so you won't scorch the fabric.
BTW: If you happen to iron on the wrong side of the interfacing (usually you'll only do that once), you can sometimes get the sticky stuff off the iron by ironing the residue off on a dryer sheet.
Whatever you end up doing, don't let a challenging first experience kill the joy. :D