Renea,
No worries. I'm a terrible piecer (I'm going back to wholecloth applique after this quilt!
) so all I did was make sure that the arc of my circle touched the 2 corners on each side. It's a very forgiving pattern & I had no problem FMQ'ing it. With a larger version, I'm sure you'll be able to do it with a walking foot instead.
For your template, as long as the plate is at least as wide down the middle as your rectangle is on the long side, you can use the plate. Too big is better than too small. For my 2" curves, I used the lid of my Poke-a-Dots container. I had so much fun quilting the design that I decided to quilt it in miniature inside my 2" crosshatching along the edges (oh! who am I kidding?! I suck at piecing so they actually ended up a 1 7/8" oh well. it's a family quilt & I promised myself I would never stress over family quilts).
I do still strongly recommend SID down the seams before you try to quilt the curves -- even with a walking foot. I promise you it looks so much flatter & nicer, and the seams are more secure, too. If you don't want that center line to show on the back, you can always use water soluble "wash away" thread to do the ditch work. It will help keep your fabric from shifting while you're quilting along the bias curves.