Gurney flaps work in that instance if the training edge is too sharp. If the trailing edge is 1/8-1/4” instead of razor sharp, it’s just drag. The original intent was to be able to add or subtract a couple of percent of lift to balance a race car when the rules did not allow suspension adjustments during a race. They are a fix not a standard.
Sadly, Fritz is no longer active in this group. His profile says, "I don't check in to the HBA very often. If you want to get in touch with me please email me at f wagoner (symbol) zia net (period) com." (I added spaces and removed punctuation from his email address to challenge the spambots.)
While routing a solid Doug Fir or Spruce spar like the picture in post #1 certainly looks super cool, wouldn't it be considering the obtainability of suitable material desirable to make a built up spar with a plywood web, Doug Fir or Spruce caps, and Pine vertical stiffening blocks?
Is it stronger for a spar to have the plywood web orientated so that the exterior grain runs parallel with the span or at a 45* angle? Could the plywood be thinner and lighter if it was at a 45* angle?