I don't remember the recipe and I'm too lazy to go back and look, but there are a number of things that will make the crumb tighter. Creating seed bubbles would help, but there's not really any method I can think of you can do too well with a bread -- that's more of a cake technique, where the idea is creaming the butter with the sugar really well. Another method would be kneading the dough, but that seems to be against the spirit of the recipe. More acid in the batter should do it, as it will set the proteins faster.

So maybe add a little bit of vinegar or apple juice to the batter? Probably instead of some of the water. Ooh, or buttermilk. That'd probably work really well. The yogurt suggested below makes sense, too.