Lighter cloth would create a much more fragile hinge. With the WBPU, it would be more prone to ripping. The thicker glass can be filled a bit but I am not going to worry about it. I'll sand it a bot to clean up the edges but otherwise I'll leave it alone.
WBPU can be structural just like anything can... to an extent. I already have fears about the strength of this one as the glass is tending to wrinkle as I slightly bend the foam. This application Is really testing the waters in terms of what can be done here. For sure this will be much lighter than a normal one. I hope it holds together. With the carbon spar and some added strength in the form of a paint stick in the wing, I'm hopeful but I do have my doubts. Another one is the strength of adhesion. If it does not hold as strong as I hope it will, the glass will pull away from the foam at the hinge and start fluttering anyway. At least I know it will not pull away due to getting wet or even the humidity in my area.