Like this? Giuseppe Bellanca was a smart aerodynamicist. Perhaps a few ribs to give it an airfoil shape rather than just a flat slab.
edit: If this is done on a Cub since the struts are separated at the wing instead of at the lower end as is the Bellanca, there will likely be some interference drag at the lower surface of the wing to strut juncture. It might be best to not run the fabric all the way to the wing.
Before I do it I would measure the angle difference between the two struts and the bottom of the wing. I think I would want a more positive angle of attack on the struts than the wing.
Good question. What happens when the flaps are lowered on a stock Cub? Do they disturb the air over the horizontal tail? Are they the cause of the different trim requirements? Does their flow disturbance cause the tail shake? Why is there pitch up followed by pitch down with the different flap positions? I do not know definitively the answer. But I suspect the airflow from the flaps flows over the horizontal effecting their operation. Thus, if the fabric covered struts are lower than the flaps, their air flow would be below the horizontals. ????
A long piece of yarn tied to the rear strut and a video camera ought to give us some clues.
Years ago Joe Soloy was trying to figure out how to get more fuel in his turbine 206’s. He was contemplating large aerodynamic wing struts that would be hollow, hold fuel and gravity feed directly to the fuel selector. Obviously he never did this, wonder why. Seemed like a good idea at the time.
I’d be more concerned about adverse effects on the ailerons. If you fabric covered the struts the only effective airfoil would be outboard in front of the ailerons due to the chord created by the fabric there. This airfoil would stall at its critical AOA like all airfoils, and the resulting turbulent flow could have adverse effects on roll control. Some engineering calculations and a test pilot could confirm or deny. Likely not worth the time or effort but fun to speculate about nonetheless.