Since you say it must be simple enough to do with the walking foot, it narrows down the designs somewhat. However, go to www.quiltingstencils.com and check out:
SC-276-06 Simple Celtic
SCL 327-05 Vintage Pinwheel
SCL496-05 Double Star
SCl-543-05 Celtic Knot Block - this might work by eliminating the double lines
Some of the Double Wedding Rings could be used as the 6" would fit
HH 34 Simple Pinwheel in the Harriet Hargrave Designs is very simple and gentle curved lines.
The spider web - SCL114-05 would require some twists and turns.
But check out the on line catalog, there may be some there that you can use.