Would this have worked (and been easier), if you had cut the trapezoid first, then routed a fixed radius cove onto the top face (it would cut air wherever the trapezoid ended) - which would have yielded a crowned trapezoid without the ramping. Then just rip the ramp from the piece afterwards on a tablesaw or bandsaw?
That is Solidworks. If you are a US or Canadian military, prior military or government employee you can get it for $20/$40 year for the educational version.
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Another program, which I’m surprised is not more popular is Moment of Inspiration 3D:
Really, really should have tried it when I was seriously looking for a 3D CAD solution — it was originally designed for use with pen tablet computers, which I’ve always preferred.
There is:
which seems to be a quite approachable series of tutorials.
The only other traditional 3D CAD tools which I even began to be successful are Alibre (which was used in the above) and Dune 3D:
(but I never got past the initial tutorial, and not sure if it would be able to do the above)
List of programs with commentary at:
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A real 3D CAD system is the way to go, for sure.
BTW, it’s called “Through Curve Mesh” in NX 
But I had to give it the old college try in CC Pro…
You have to make the workpiece quite a bit bigger to accomodate the model.
This is 48" x 48", and 3.5" stock height.
I extended all 3 top radii to a 43.6° tangent, and then measured the height. Using the difference from the total height, I constructed the long curved base solid. So it’s 2 pieces, the larger base, then the rounded top. The bottom section is right at 2.5", plus the center section is 1" taller, so it worked out to 3.5" overall.
The 3 small circles are gage blocks, the ends being 2.5" tall and the center 3.5" tall. So there is an imaginary 1.5" base in the simulation.
The 3D model
The simulation, with a disposable pocket toolpath to display the remaining shape.
Bozo_curved_surface.c2d (1.3 MB)
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It was a typo. “Loft” instead of “lift”.
And if you look at my second post with image and stl file attached. It automatically curved it and did not leave a sharp edge.
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I saw the image, and it looked good, but it looked like one end was too tall. I am not sure if that was just the perspective or what.
The compound curved board is very similar to this:
I thought it might make a nice back for house numbers. Cheers.
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OMG - it’s $800!
Drop in a $10 clock…
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I guess one could make a clock if wanted. 
Merry Christmas
You can get Solidworks 3DEXPERIENCE for Makers, the same cost. You can run it locally on you computer or through a Web interface. Basically the full blown package without some of the features you probably would not use. Used Solidworks years ago and just got this. Great program!
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