How about this? I'm pretty happy with it now. Raised the rear deck a couple of inches, made it a little longer & sloped the rear panel more. I think it looks like the back of an old Chriscraft, which is kind of what I'm going for.
Can't figure out how to mock up the spare without having the whole house of cards come toppling down but I think it will work either way. The bottom edge of the sheetmetal will be lower, maybe with some belly pans.