When I make diaper cakes, I roll the diapers,secure with rubber bands, and put them into pyrex round white baking dishes that are graduated. (this way I have the perfect size for each tier) I can pressure fit them inside, then tie a bow around them that covers all of the rubber bands. It uses a lot more diapers than your method, but then, they are part of the gift, too.
I think your method looks AWESOME, and the next time I will have to use some sort of tube for the center
I am smacking myself on the head and asking, why didn't I think of that?
The mommy and little man will love it! Great job.