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Update, Summer 2019:
I have just a couple aerodynamic projects planned for now: first, adding the front wheel strake fairings from a 2018 C-max hybrid and second, air curtain ducts from a 2018 Honda Clarity (will be easier to retrofit than the Hyundai Ioniq ducts I have on hand).
But the big project I'm working on right now is accurate coast down testing, to see if I can calculate drag coefficient with some precision. I've downloaded and read several papers on the subject, relevant chapters in two textbooks, and have worked through most of the math. Last week, I downloaded a topographical map and identified several stretches of road that are candidates for a test site with minimal/no elevation change. I'll drive out and check them out in person this week. I'm also about to order a Kestrel 2500NV ("night vision," since I'll probably end up doing this at night when winds are calmer and traffic is lighter) wind meter and wind vane (made for the 4000 series, so I'll have to modify it to fit the 2500). That meter will measure windspeed to 0.1 m/s accuracy, as well as temperature, barometric pressure, and wind direction with the vane mount. Using that data, I'll be able to calculate the actual wind speed of the car.
For the overall force calculation and rolling resistance force component, I'll be using the corrected mass of the car. Chapter 14 of Aerodynamics of Road Vehicles suggests a formula for calculating this using the inertia of the driven wheels and driveline and non-driven wheels and their respective radii. I looked up the weights of the stock wheels on Priuschat, the tires I have on the car (thanks Tirerack!), and averaged the listed weights for various replacement front and rear rotors, wheel hubs, and the front axles and used the dimensions of all these to calculate the inertias. The overall correction factor results in an effective mass of 102.19% of measured mass. Next, I'll use the scale at one of the two recycling centers in town here to measure the mass with me, the wind meter/tripod, and a full tank of gas.
I'm going to write all this up like a lab report when I'm done so if others are interested they can read it.
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