Update, although kind of late lol:
Went to Home Depot to buy some OSB board to cut up and get the project underway...then realized that a 4 foot wide board won't fit in my car. So the bungee cords in the back of my car came in handy, I was impatient:
Got home with the 2 4x4 boards, they're a good size for the undertray:
Screwed in a few boards for the border just so I can contain everything, assuming something spills or drops:
Cut a few rough boards out to raise up the curved edges of the undertray, so I don't use so much filler material and have a few references for when I actually make the mold.
In the third and fourth pic, the back left corner border isn't flush...I used a bit of filler material just now to fill it in and get used to the material. I think I ended up using too much hardener because it hardened in under 10 minutes, and I found out I have a pretty small window in which to mold the filler (unless I use less hardener, then it widens a bit)...too soon and it is sticky and I'll have a hard time releasing it, and too late it just won't mold.
Tomorrow I'll start laying the actual mold, I might do it in sections because the whole thing is too big to work with.
Course, I could tape a large piece of paper or thin piece of cloth around the entire undertray, press it into the filler material when wet, untape it to remove the tray and let it dry...which leaves the impression but doesn't dirty the tray, and peel it up afterwards...I'll have to see.