"Make an pattern out of poster board, transfer to a heavier cardboard . . . "
Then hot glue the heavier cardboard pieces (or better yet, foam core) together. Brush both sides with fiberglass resin mixed with hardener. Once cured, cover both sides with resin and fiberglass mat. When cured, trim a bit to fit with a grinder/sander, place into the opening, and glass the seams where it meets the body sections.
Spray with a coarse "chip guard" to hide the rough or irregular fiberglass surface, and paint body color.
Done . . . and pretty professional if you take your time a bit.
Luck,
TC