You can easily use a rectangular as long as it falls in the divisible by 4.5 and 6.5 so you can get your borders on without being creative.
I believe in dragonflynana's round robin there is a rectangular block that was sent around
http://www.quiltingboard.com/picture...n-t224012.html
check through there she posted her center early on and there are now two rounds on it I believe.
Any questions here can be answered by either Jaba or Myself so feel free to ask away. She is way on top of things, me I am trying hard