Do you currently pronate on the take back (face your palm downwards)? I found this, and the timing of it, to be the trickiest part of the Federer stroke.
Instead, I like Del Potro as a model because he doesn't pronate on the takeback. It's a lot simpler and easy to learn, though I don't think it generates the same power or topspin. It might be a good transition stroke.
Anyway, one thing I focus on is my elbow. I think of my elbow as whipping my forearm forward and outward. This visualization really helped getting straight on contact and hitting through the ball cleanly.
Also keep in mind that when drop hitting a ball, you gotta drop the ball really far away from you, otherwise you will be building a bad habit. Maybe this is why Federer starts rallies with an underhand squash shot instead of his regular forehand.
BTW, I would add Berdych to the straight-arm club. Although his arm is bent at contact, his racket accelerates forward with a straight arm, then he bends the arm into contact. I don't recommend him for emulation though, because this is a more complicated stroke.