I know you are well down the design path. But I haven't been able to keep up.
Two things.
I found this on another site. It's a way to drain and fill the water tank I thought was well done.
G503 Album - 1946 Bantam T3-C #9466
Also,
On the fenders. I saw the way you mounted the fender to the sheet metal. It looked like to me it might cause the metal wall to flex when you do some bumpy roads. Essentially the fenders might, ever so slightly "flap" like wings on that wall.
If you tacked the top to the center support you could eliminate a bit of flex there. It's just an observation.
I marked the supports in red.
