Kevin, Google Replacing front rv caps and building teardrop camping trailers. There are some very informative videos on the under layment and attaching the filon to it. It is amazing how you can bent the 1/8" plywood around some pretty tight radius. I am getting ready to tackle my front nose in about a month (I lost a clearance light reflector and water got in). I am going to use a 1/8 birch plywood. Like you I am going to make additional supports including one in the middle out of 3/4" plywood cut to match the existing radius and laminate them together in pair to get extra bearing width to attach the plywood to. My filon is 8'x8' so it will require 2 - 4'x8' sheets of plywood for a backing. I refuse to use the cardboard again. Might even cover the plywood under layment with an epoxy to prevent water intrusion.
