Not my best, but I kind of like it.
As Straight As…
Hermione Granger is as straight as the ridges that grace a dragon's back…
As straight as the tail twigs of especially old brooms…
As straight as the meniscus of graduated cylinder when measuring potions…
As straight as the cat, Crookshank's legs…
As straight as a woman's figure…
As straight as the curve of a cauldron…
As straight as Hermione's bushy hair…
As straight as Ginny Weasley…
As straight as puckering lips…
As straight as twining bodies…
As straight as Hermione's face as she lies to Ron for the first time…
As straight as rivulets of tears and answers to questions.