Repost from the G1 Insight thread, I wanted this with my mods here:
A few days ago I went around town and picked up a bunch of random signs, and used them to construct my rear pan today. I cut up the sign holders to stick through the corrugation and give support, as well as hold the individual pieces together, since I'd be making it out of ~6 individual pieces.
I tucked it under the middle pieces, plus a few zip ties to support structures. I cut out around the exhaust and bent the coroplast up in that area to keep it from catching air. At some point I'd like to do a complete pan front-to-back using only a few pieces, replacing the stock panels, but this is an improvement as it covers some cavities.
After driving it around a bit, I found it rattles near where the exhaust pokes out in the back, so I'll need to figure out something for that.