Wind tunnel would be the ideal for this, if you can get one. If you can't....
Thinking outside the box, this might be a good project that will save a lot of money in engines and may be more consistent.
https://exploration.grc.nasa.gov/education/rocket/rktstomp.html
has a generic air pressure launched rocket system. Basically can set this up like a piston system. What I am not sure of is if you can get a rapid pressure release system for launch. You can pump up to
the same pressure for every launch, release, and measure either altitude or time of flight. I am thiiiiinnnnkiiiinnnng that these won't go very high, they would be very very light (only a nose cone, body tube, and fins) and could be featherweight recovery, reducing the variability in descent time. You will need a soft grassy field, as these will come in balistic. Put a bulkhead in behind the nose cone to hold the pressure, and you can use the same rocket body and just swap out nose cones. To make sure the greatest effect on drag is the nose cone, experiment with the minimum fin surface area required for stability (may not take very much fin area.) Given the low mass of the rocket, I suspect the drag will slow these down in a hurry.
The thing I am not sure about is the pressure release valve, has to be able to nearly instantaneously release the stored pressure in the tank to the rocket to prevent variability. I am thinking you don't need a launch rod, you use a piece of metal pipe just less than the inside diameter of a BT-20 rocket and fit the rocket over the pipe. Max velocity will be reached at or before the rocket is off the pipe, if it isn't stable by then it is never going to be. Rather than a bike pump, 12V tire pumps are pretty easily available and cheap.
You will need a tank and a pressure gauge and a rapid release valve.
Advantage again is cost of engines and ease of flying flight after flight after flight.
Addendum: Actually an easier measurement device would be (and again, safety considerations come into play here, this is NOT NAR safety code) to launch at an angle and measure distance traveled. NAR code restricts angle to 30 degrees, but I am not sure NAR code applies in this case. I believe 45 degrees gives you the best bang for your buck. Again caution adviced, these would be featherweight but the nose cones are kinda pointy.