This is how I built a Coroplast tailbox to fit my Lightning P-38.
A tailbox is what its name implies- a rigid box that sits behind the rider, usually intended for carrying cargo. As a side benefit, it can improve the aerodynamics of the bike slightly.
I was inspired to use Coroplast by Bill Volk and his Coroplast Craziness page and Ed Gin's Fairing Seminar. What I discovered is that Coroplast is an incredibly easy material to work with- it's cheap, easy to cut, tough to goof up, waterproof, easy to clean, and available. Since it's commonly use for outdoor signs, most sign making shops will have it in stock. If you're really cheap, wait until the next election and volunteer to clean up candidates' signs afterwards. This usually yields many useful large pieces at no cost. You can use these for interior panels (like I did) or, if you don't mind the printing or the colors, on exterior panels.
The one deficiency of Coroplast is that it can't be glued easily. Epoxy, silicone, hot glue, and rubber cement have been tried with varying success. Coroplast's Website has some good information on what works and what doesn't: Bonding and Adhesion of Coroplast. Since the tailbox's parts move and vibrate, most bonding methods will fail in short order. My building method is to stitch the edges together with plastic zip ties. These are cheap, easy to find, and strong but not particularly elegant.
The dimensions on the following pages will allow you to build a similar tailbox. It's designed to fit a Lightning P-38 with a rear rack. In fact, I built the original simply by eyeballing most of the pieces- I didn't have any detailed dimensioned drawings before starting. The curved sides give it rigidity and keep it pretty quiet. With some thought and ingenuity, it could be adapted to most recumbent designs. Heck, with a little reworking, it could fit an upright bike!