I love all the ideas here. So here is a little math for those who want a square in a circle or a triangle in a circle. For the square in a circle, remember the diagonal of the square would be the same as the diameter of the circle; so for an 8" circle the sides would be 5.65", & 8" diagonal. For a an equilateral triangle (all sides equal)in an 8" diameter circle ~ 6.93" per side of triangle... easiest to just follow this link & print out the shape on your printer.
Here is a link to a lot of shapes inscribed inside a circle, which you could print to scale to fit your filter.
http://etc.usf.edu/clipart/galleries...p?page=2&term= You can trace over a light box onto your filter or tape it up to a window for tracing. You will also find stars in a circle, & other shapes. Hope this will give you another design for your new favorite circle template.
For those who don't want to sew a curved edge that must match up, just do it as an applique with fusible & satin stitch.
Here is a video to a class I attended @ a quilt show several years ago. She will @ the Sacramento, CA show this weekend @ CAL Expo doing this free class.
http://quiltingwithcharlie.com/quilt...structions.htm
She does a nice & simple method of sewing circles together to make some fun quilts. Fast & easy.
Best wishes, Cathy