So I've done this on two vehicles, 2009 Nissan Versa:
These showed a documented 2.3% gain in MPG measured over 5500 miles.
I did it again for my 2013 Dodge Dart, two versions:
MkI:
And then MkII when MkI showed a propensity to fly off:
In all instances I modified existing hubcaps. I purchased hubcaps that had the most sympathetic shape and then filled in the spokes.
I'm now contemplating doing this for my current car, 2017 Toyota Corolla. But as I've already done it 3 times I'm looking for the easiest way possible. I've purchased some flat-ish hubcaps for cheap, but really not relishing having to fill the spokes again. And, the last Dart versions turned out so well my standards have gone up, unfortunately...