Try this see if this works
In the front view create a plane does not matter what size now take the segs lengh and width down 1 then convert down to a poly, go into edge mode, and select the four edges on your plane, under edit edges, choose the create edges from selection now put the dot in linear, and click on ok
on the main tool bar choose the select by name, you should have two objects in the list one is the plane and the other is the shape, if you try and extude it then the spline will encompass the plane.
this should work on whatever your trying to model,
if this is not what you mean then please throw up an image or give more detail on wha you're modeling!
