For the second shell, I built one that can't fit over the battery pack. A small, flat magnet proved sufficient to keep the chassis down at the front (though it's not secured and can fall out easily; that, and the shell alone still pushes it down anyway). I chose a typical diesel with American elements, mainly the cab design and handrails along the body. It was done both for the nose shape and as a homage to Supes' no eye mod for Zack. The orange livery with the grey roof and chevrons was envisioned before the loco was built. Have to say I like it even more than the "not Rail Blue" one.
Once I could perfect a design for a scratch built "Thomas drive" chassis (complete with activation tab), I could use these shells with it in such cases where they don't fit over the 2x AA pack.