Ahh, I should have mentioned I need the collisions to be on the outside, since the AABB is actually the bounds of the gridmap, I want to automatically generate collision around the edge so nothing can leave it. I'm making a start:
var boundsbox_node = $BoundsBox
boundsbox_node.get_node("Left").shape.extents = Vector3()
boundsbox_node.get_node("Right").shape.extents = Vector3()
So I need to set each of the collision shapes (Left, Right, Top, Bottom, Front, Back) to match each side of the AABB?