Hey, Guillermo, I got one of these (Ironhide) from Ricardobot a couple years ago. What I did to it was cut a piece on the back of the head, remove it from the clip-on attachment and sand a little in the forehead, and placed it inside the vehicle, behind the windshield. It's not cartoon accurate, but looks really nice. What you could do is cast them on a smaller size, so they'd fit inside the vehicle by themselves. I made Ratchet's head out of a Bumblejumper's, and the size fits perfectly. I'll provide some pics of both tonight once I get home.
