Totally agree with Alan, head first!
It looks odd if it goes with a tail, but without a head. Then again, we don't want a head constantly looking left and right all the way
Another method you could use is to mount a servo in the middle, inside the body (can it fit?). The horn is to be secured onto the body, leaving the servo free. That way you could use the weight of the servo as weight.
For additional weight, best to use common steel screw and nuts, attached to the servo. Steel density is about 3x of aluminum. You can save the bracket for the head!