Okay, that's what Paul is doing right now, today! HE was able to salvage the original floor -- well, all but the front piece, and got some old plywood from a neighbor. He laid the originals down on the "new/old" and traced around it, then cut it, and laid it loose inside and tweaked and marked and trimmed it.
He had it in and out of the trailer several times.
Finally he even added bits of cardboard where
the original was too small; then he traced it onto the good ply (being sure which side went UP) and cut carefuly with a jigsaw. The big back piece needed only one cut, for length, as it fits on the amerigo
floor at 4 feet wide, so no other cutting. Nice. .
BUT...you can make patterns out of cardboard or even heavy paper...You can use brown grocery bags if you have them, even. tape it together if you have to. Remember which direction and how big your ply is...(most is 4 x 8) -- there's really no math involved, just make your pattern oversized to start with, and nip away at it in place, kind of.
I hope this is clear enough? It's easier in person...
Let us know how this goes!