......Hmm maybe embroidery floss, it’s black on black do it wouldn’t show. Ohh I just love the Quilting Board!
In the early days of Eleanor Burns and her QIAD projects, the go-to was to use embroidery floss.
Without knowing what the centre of each of your 10" blocks are, it's hard to make suggestions that are do-able.
One possibility, would be to quilt a box, one inch from the edge of your ten inches which would give you the 8". Or of course, could go with 1-1/2" all round or whatever works best.
Or if it is a pieced 10" block, use the piece work to help you decided where to quilt.
Or if the centre is a printed picture or embroidery or even the piece work, you could echo stitch around the shape.
All those, and more could lock down your bigger than 8" opening, in order to get the quilting within the parameters of the quilt batting recommendations.