Your first question states that you wanted to go larger than crib but smaller than queen, and now you have stated you want to keep the queen and be able to go to king. Sorry, but I'm confused.I've been out looking at my Grace SR2 frame trying to come up with something. The only thing I can think of is to contact Grace Co. and see if it would be possible to purchase another section. At least then you would be sure that the parts would be of the same size and smoothness.
When I'm doing crib quilts I simply center the quilt in the middle of the frame and have enough room on both left and right sides to allow my machine to move freely without bumping up against the end rails. As I said, your two sided question leaves me confused.