I think I can envision how to do it using Ronda K's hints. Miter both the front and back borders without any batting involved. Then slide in a strip of batting on each side, zig zag it to attach it to the batting for the main quilt top and then quilt it, one side at a time. The border batting need not be mitered. You could do the sides first and then the top and bottom. I'm strongly tempted to just send it out for quilting, especially as my machine started acting up.