What about just filling in the gap between the wheel and the wheel well, make it solid, and leave the minimum clearance for turning. Then put some sort of rubber stripping around the rest that can basically seal it when the wheel is straight but will flex enough for the wheel to pass through it.
At first you might think it wont be nearly as effective because it doesnt cover the spinning side walls of the tire, but I bet most of the turbulence created from the spinning tire is actually from the tread part of the tire that is perpendicular to air flow.
This coupled with some hub covers should be very effective, and possibly even more then a full cover because it wont increase frontal area and might have better laminar air flow.
Could do another tuft test with a cardboard cutout to see
