If you have a 50 km/h downhill section, that would be a good place to measure. Aero resistance roughly quadruples with speed. Ted Hohls comment below is better formulated. At 50 km/h you can be sure the major resistance is wind. At 25 km/h rolling resistance still plays a major part. As a % of total it gets less important as speed rises.
Simply rolling down that hill, starting from 0 speed or a fixed speed of the speedo, would give you an idea. It is important to realise that other factors like wind and temperature can mess up the results. If you measure on different dates, it's good to have 1-2 control runs with a setup you know. If setup x does 0-40 km/h and setup Y does 0-50, the later is best.
There are 3 things that factor in on overall aero resistance.
Frontal surface. Less is mostly better. On your upright you are about 180 high, perhaps even 190 from the ground, and about 50 cm wide.
The smoothness of the shape. Even a small edge can cause a disturbance, and vortexes. A Vortex causes more drag. As an example the difference between a modern glued in car windshield or a 1970 one with a rubber strip and chrome trim.
Wetted surface. The surface of the object that is "wetted" by the air. You can get a very elongated shape, that is smoother to the air. But that same longer shape, can create more surface friction, of the air flowing past it. This works together with the previous point. A cylinder upright, has less wetted surface than an aero, symmetrical wing shaped profile around that cylinder. If you elongate that wingshape even more, eventually you will reach a point where the total drag does not go down, but starts going up.
A good place to start with aerodynamics is Julian Edgar, he had several youtube video's on car aerodynamics.
With your upright, you are roughly putting 50x180cm in the wind. That is 9000 square cm. A Snoek velomobile is 80 cm high and 70 cm wide at the bottom, but only about 30 at the headfairing. I would estimate that to have roughly half the frontal surface.
If you want to improve your upright, first look at how they improve UCI approved racing bikes. That is where the money is, and where most research is done.
Mochet experimented with tailfairings on upright racing bikes in the 1930ies, Then went for the recumbent position and eventually came to fully faired recumbents.
This is also a very good tread.
Fairing an upright is basically like fitting a spoiler on a VW bus to make it faster. It can be done, but there are more efficient vehicles to be found if you want to get the most speed out of limited horsepower.
When racing lowracers, the common experience when adding a lowracer tailfairing out of Fibreglass or Carbon Fibre, was a speed increase of about 10% at speeds over 30 km/h. Those fairings fit very nicely to the riders profile. http://www.m5-ligfietsen.nl/site/EN/Models/CrMo_Low_Racer
I am pretty sure anything shaped less perfectly, and less smooth will provide less performance gain.
If you want to improve speed on an upright bike here are my tips;
- Drop the touring tyres and mount some fast racing tyres decrease rolling resistance from 50 to 20 watt
- Fit an Aerobar
- Wear a onepiece TT suit