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For the tail cone,a design has been tested with 25-degree top,10-degree sides,and 10-degree bottom diffuser,which showed lowest for all of the group tested
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I had seen that illustration in other threads, but was not aware of the 10 degree sides and didn't notice that the last item was the lowest Cd either (I was concentrating on the 3rd and 4th from the top). I also do not see what is different between the last two items. What does the cross-hatching represent?
I think the taper on a boat tail would be pretty easy (no need to compromise with that square box), but getting the transition smooth like the NASA van would not be. Hope that doesn't hurt too much. Also I have a feeling that adding the bottom part of the cone is not worth too much without also having a belly pan on the trailer. Or maybe a dam at the front of the trailer and skirts on the side, to exclude air? A belly pan would be pretty easy on this trailer, just hang some plastic roofing under there. Skirts and dam would not be that tough either, provided I find some material that can take a beating (I was thinking of plastic baseboard molding).
Right now in the Ram I have a home-made half-tonneau. I realize the gap is a big problem and probably is a lot of work to fix, which is why I'm thinking of just winging it with a lip on top to separate the flow for now. A vortex trap or two might not be too difficult, for what it's worth; but I'm guessing they don't help unless the Ram has a high cap.