With your remaining bars, cut slats to create the x direction. Cut them as accurately as possible, because they will most likely be off (I had to do quite a bit of sanding to get everything right). Measure and glue them in (see my picture for reference).
tips: have a friend help glue while you put them in. Mark where you're going to put them BEFORE you actually put them in- this'll save a lot of time in the long run.