Today I started gluing things together!
Decide to start with the long sides first. Decided the fastest way was to clamp both sides at the same time, keeping them separate using some plastic.
Kept going with the smaller rails, but kept these separate with parchment paper.
The plastic seems to have kept the Great Stuff from setting up, but the parchment paper didn't have the same problem.
I figured out that the scrap cutouts from the wheel wells were the exact shape as the front curve. Decided recycling them was the best thing, so I cut the three I had in half, and cut out a couple more.
After the GS residue dried, I tried again, but this time without the plastic. Definitely worked better this time!