Thanks alot for the input you guys.
It is a “poly” shape, so yeah, couple thousand words of pics should help.
you can see the wrinkles in the original underlay, a felt-like fabric. I bought another of those for padding and support. other projects have bubbled to the surface, I’m glad this info will spur it back forward. I’m whipping up a 10ft arched bridge outta 2x12 pressure treat, and cedar slats, be nice to have water under it