Cardboard was put down below the mulch as a tentative barrier. Because the barrier wasn't in place quite long enough, the grass was still alive on planting day and ended up growing through over the course of the summer. Lesson learned!
On planting day, the bed looked great! The few bushes we left from the previous garden were healthy and all was well.
A path was marked so that where seeds were planted, they could be located before growing big. Squash ended up growing fine on the edge of the rainscape but we believe it was too hot for other vegetables to survive.