Well, as mentioned in this "Road creation width
" related thread - "the scale is not progressive"
, but it might be possible by dividing it into separate parts.
I would suggest you to try split it into three generators:
(1) For sides, using two "border" splines
(2) For boards, where creating and using a "centre" spline and a closed spline (made from "border" splines) as a Clipping area
would be recommendable
(3) For support structure, using the "centre" spline
Hope that helps.