I have that metal rod thing that Jan is talking about, but what I do in my borders works just as well, at least for me. I use a book mark and just lay it against the last SID I did around my inner border. Then I just stitch along the side of the book mark and move the book mark as I go. It leaves me with a stitch parallel to the inner border. I have different width book marks and have even just made a "book mark" out of s thin card board that worked.
Let me see if I have a quilt picture that shows the result. I quite like it, and it might work for you. (I consider myself an experienced beginner and I like things simple.) Okay, the seam line can be seen on this quilt. I use a batting that says I can quilt up to ten inches apart.
Dina