The following images were obtained by placing a slice on the platten of a scanner, and scanning. The second image is an enlargement of a portion (indicated by a red rectangle) of the one immediately below. Well, I think I know the answer already, but next time, for bigger holes, I may try a second stretching and folding after the rise, and add a second rise after that. After that, I will see if I can apply a similar technique to sourdough.