The applicability of two-dimensional flexible solid wall test sections for three-dimensional testing is theoretically investigated for linearized subsonic flow. The method uses known interference velocity distributions along a targer line (derived from a method of images for this particular study). From these, wall shapes are calculated that reduce or even eliminate the axial gradients of wall-induced upwash and blockage along this target line. The examples of residual wall interference presented, show that these gradients, as well as the overall blockage level, can substantially be reduced by means of quite acceptable wall displacements and test section lengths, even for half model testing.