I used some foam and clay to make the shape I wanted to close off the inlet ducts.. I covererd that with a layer of saran wrap so that I could remove the clay and foam. I then layed up 2 layers of fiberglass (West Systems). Once that set up I separated it, removed the clay and foam, made up some epoxy and flox and then fastened it back in place with clecos until the flox set up.
