Surprised you cut the whole bottom out of the cowl, it could have been more enclosed seeing how the engine head doesn't stick out the bottom. Good Idea on the wing tube, you will find out why when you put it together at the field, it's a PITA and need two or three guys to do it.
My plane is a 1/4 scale balsa kit made into a super cub, wings on using the tube as it's only support making it easy to put the struts on by one guy. You might even like making this tail wheel assembly.
Leroy