Tapered Wing Providing low Coefficient of Lift
By tapering the wing you have in effect a 3D case even if the wing is sitting between wind tunnel walls. Lift varies along span almost as if the wing would be in free flight with twice the span and the long chord at the root. Since tip chord is only a quarter of root chord, that full wing would have 0.075 m² area with 0.5 m span, giving it an aspect ratio of 3.33. Below is a figure from Sighard Hoerner's book Fluid Dynamic Lift (Chapter 3, Figure 4) which shows how the local lift coefficient varies over span for wings of different taper ratio. Your wing in free flight would be ¾ between the second and third from the left.



A 2D case is only possible with constant chord as all of the wing would produce the same circulation (lift times chord). With the varying chord along span only the shorter end will reach the maximum lift coefficient of 2.3 locally, but at a lower angle of attack. You still get more lift with your wind tunnel walls than in free flight, but with a taper ratio of 0.25 not much of the 2D case is maintained.

The higher circulation of the long chord end produces extra circulation on the short chord end and lets it stall at a lower angle of attack when the root has only reached half of its full potential (see figure above where center lift coefficient is only half of the tip lift coefficient). The reduction in maximum angle of attack from -13° for the 2D case to -10° also shows how much potential lift is lost.