Joe , we made the exact same mistake about the canvas shrinking as it dried and pulling away from inside corners... Although we initially glued it in place and fit it so nicely square , as it dried it all pulled away from the foam leaving an air space ... which here results in canvas rot

( note: NO rot ever found with “attached" canvas ) Eventually we were so concerned we modified the roof and eliminated any inside corners , so it can now shrink as tight as it wants to ...