Personally, I don't see the rear wing affecting your thrust. Thrust is a product of the propellant mass flow and the velocity of the flow out of the cylinder, which isn't necessarily affected by the wing.
Although, I do have another somewhat "out there" idea for you... you wouldn't happen to have access to a
Schlieren Imaging setup would you? I do propose this as a half-serious suggestion. If you have a local university that has an aero department, they probably already have one set up. If you can network with them, they may let you come in one afternoon (if you catch them at the right time of the year, they may not be terribly busy and they're like us... they enjoy seeing projects like this). It's worth a shot, and it is a really interesting process and you would learn a lot. Not to mention it's a very powerful tool for visualizing flow fields, like is shown in the video I linked. If you were able to get it done before you finalized the design, it would be really nice to see what the flow field resulting from the CO2 cylinders on their own looks like. That way you could potentially make a better assessment for how to design the rear end.
Even if you aren't able to find a place before the final design, it would be interesting to test it after the fact just to see how effective your design is. At the very least, if you're considering going to school for this it would be a good way to research schools.
