Using a walking foot, you could make vertical lines following your rails. I would go with diagonal lines in your slanted section.

Have a look at Jacquie Gering (spelling?) ....she has some great advice and techniques for dense straight line quilting that includes diagonals etc. I'm sue you would find some inspiration and technique there. I use her books and Craftsy classes often.

Pat yourself for the stitch in the ditch part done, no way would I rip all that out. That part is done, you'll figure out the rest. Sometimes walking away for a day or two helps.