D9P? I get the 9-patch part but am blanking out on the D.
I found stitch-in-the ditch to be somewhat difficult, perhaps because I am a perfectionist. Even with an edge foot, I found it difficult to stay exactly in the ditch or precisely on the edge.
My suggestion for an easy quilting approach would be a "wave" pattern using a walking foot. Basicacally this would be just wavy lines. You would start in the center of the quilt and work you way out on each side. No line has to exactly match the line before; in fact, a little random variation would would make it very natural looking.
You could add more quilting by cross-hatching the whole thing. That is, once you have lines going one way you can turn the quilt and make lines going diagonally across.
Alternatively, you can sew one line down the middle one way and then turn the quilt and sew one line down the middle the other way. This would actually stabilize the quilt sandwich better from the beginning.
I think the crosshatching would be especially effective if you work from one corner of the quilt to the other. Because you would be sewing so much on the bias, I would just be careful to heavily starch the backing fabric and top before layering to ensure they don't stretch on the bias. For diagonal crosshatching, I think I would sew from one corner to the other through the middle and then from and to the other corners through the middle first -- more stability.
The beauty of doing wavy lines is that you don't have to worry about anything being perfectly straight or evenly matched. You can just relax as you sew. Variations will look natural. Plus, you don't have to worry about stitch length because your feed dogs are up and you are sewing as usual. I would really want to use a walking foot, though, to make sure the top is moving evenly with the bottom layer.