Did you pin the sandwich or baste or glue baste? If it is pinned, whatever you choose may force you to reposition some of the pins.
If you decide to do diagonal lines (and I also suggest using the wavy option since it can hide a lot of minor oopsies) You have a lot of reference points that are quite close together. Just quilt the line diagonally, one square at a time. I would start with making a big X from corner to corner. That would certainly help to anchor everything.