Countdown: 8 rings made, 22 to go. That's more than one quarter complete! By far the easiest way to do this is to attach the rings to the center without the corner squares and add them last. I think that was in the Hopkins DWR book. It makes for a lot of set-in corners, but still is easier. It also makes the pointy corners of the center block come out cleaner.
The big problem is avoiding stretching. This thing has bias every which way and is not stable. I don't envy the longarmer who will get this baby. With the scalloped edges and the stretch, she will have to be on her game.