I agree with Bill - the sharp edges of the wood will create more drag, particularly the side edges ahead of the front wheel arches.
If the only reason for that wood is to have an easily removable platform for testing the lower rubber(?) air dam, I would trim the wood area down as small as possible, and then for the purposes of testing, use duct tape to smooth the transition from the platform to the bodywork behind & beside it.
I agree with you that strong wind is a disturbing factor, even if I prefer a strong steady lateral wind to a variable head wind. But since air dam is there to «split» the wind head on, better on a windless day for tests. My second test gave me peace of mind. But I was surprised distance was greater under a strong lateral wind.
Moon disc hubcaps : I thought first it was not that good (expecting too much), but on second thought, a 2% improvement for such a small modification is good considering the avg speed of only 40 kmh ( fast acceleration to max speed and then gliding on a long flat to a stop). At 100 kmh (65mph) it should give a 6% improvement.
Air dam : Since the underbody of my smart has a double floor quite smooth, basjoos must be right about the inutility of an air dam except maybe a little skirt in front of the wheels, as 88HF suggest. Well basjoos, I am dreaming about a full tear drop boattail ... but difficult to realize and would modify the shortness parking facility of this car !!! I was considering more CO ZX2 advice on modifying the front of the car to reduce the back drag.
Flat air dam : This quite big flat air dam seem to increase somewhat the frontal erea. The sharp edges seem to create more drag, has said MetroMPG, so trimming the edge of the wood and duck taping for smooth transition would almost certainly improve ( I could be surprise there). But if the air dam has for function to divert the air flow to the side, a flattish one won't help. Looking at «old reliable» the air dam is rounded from center to the side. Mine is slanted to the bottom, pushing air under the car. It seems to be of no use since the plastic front has more rounded sculptural form without the dam.
Well it make me think that small adjustments may make big differences. As skewbe said : « That's a tough aerodynamic case. It's almost taller than it is long. But it is so light that aerodynamic improvements (especially @100kph) are real critical to improving your glide times. »