Many conventional experimental tunnel test sections are enclosed by metallic cylinders. For optical applications, a relatively inexpensive approach is to replace the metallic test section of the cylinder with a plastic or glass tube. This paper describes several ways to neutralize the cylindrical optical power, and hence the astigmatism, of these transparent tubes. One approach is to enclose the test section in a rectangular tank filled with the tunnel fluid. The outer radius of curvature of the tank neutralizes the residual cylindrical power. Another approach is to use an external cylindrical lens without the tank surface modifications. Finally, an external lens can be employed when the tube-fluid combination is used in air without the tank.