Jakie, Thanks !!! Still have some details to work out ...
Norm , That does make sense ... To let the floor support the wall ...

May have to make another drawing ...
I was thinking about pre covering the inside of the foam panels like we did with the little one ... I had thought about either painting the inside or maybe even a wallpaper type covering ... But I still don't know ... Doing all that after , covering between the ribs would be a PIA , but allow a better joining of the foam to the ribs ... The plan was to make the entire wooden frame over a couple foam bulkheads , like on Tink's Dad's drawing , then just add the foam ... Each rib would be placed so they would overlap the seams of the foam panels , or fall out in the middle ... so I need to have a clean glue to foam join where the vertical panels join together and at the middle ribs ...
Since the stringers would float on top (inside ) the ribs , there would be some room to fit the covering behind them ... Either that or make some decorative interior panels and screw them over the stringers ... as an interior wall .... sort of ... Sorta' thinking like the inside will look like a conestoga wagon or the inside of a dirigible ...
I admit spending too much time pondering the Jules Verne trailer ...

Going for a Vintage interior look ... For what that's worth ...
