When it comes to a shape that is not readily available you may have to make it yourself. One of these shapes is a taper, cone, or bevel. As far as I can tell their are two way to make it right off the top. 1) Horizontally: aka laminating. I have not tried this yet but plan to some time soon. 2) Vertically: cutting the profile of the shape several times aka ribs.

The second is the way I went with only that mine is in a circle pattern. I filled the fins with bondo let cure and then repeated. After that I gave it a series of sandings 60 grit to 800 grit. Then I filled the missing spots with wood filler and repeated the sandings. I will have to do this several times to get it all filled and smoothed. The problem is no matter how much this is done it is still not round! Its round-ish but flatter between the fins. I dont really know how to build it up other than just globing a massive layer of bondo and grinding it down till I see the fins poke through. As a side note, I have the piece on a mandrel and chucked into a battery powered drill.

The bondo does make the piece heavy though. I have seen pics of people using 1/4" foam in between the ribs to keep it light while still filling the void. Im not sure as to why though because bondo will "eat" the foam and thermoforming will do the same. What kind of fillers are used on this and what fillers would you recomend other than bondo and wood filler putty? -D

Edit: Here is a pic of the part before the bondo...