My original plan was to have the last spar sit on top of the roll pan, and up next to the end of the floor. After I look at it now though, and since you mentioned it, it does look like that would cause water to run back down into the roll pan and the end of the floor
Hummm.... gonna have to think about it a bit more. This is one of those small details that I fretted over when working out my design. I got to the point where I decided "I will work that out when I get to it".. now it's bittng me in the butt.
I guess I could extend the floor out to meet the end of the roll pan. It would be supported by the roll pan, which will be screwed up through the end of the floor..

The last spar could then rest on the top of that?