floor gone- now what?

Folks,

The floor of my open fishing boat is soft and needs replacing. The design is a one-piece liner with "plugs" in it for hatches to engine compartment and fishing wells etc. If I cut the entire floor out - a piece that would be 10'x18', I could either use it as a "plug" to create a new piece or remove the rot and re-support the existing "skin".

My question is how best to put it back together? Should I try to build a lip around the edge of the floor for it to lay down on. What would you do? I need ideas so that it goes in right, feels strong, and has a way to stop water from avoiding the scuppers and getting into the motor bay.

thanks