There are lots of reasons you might like to have an enclosure on your bike: they can improve your aerodynamics, help with thermal regulation, serve as structural attachment points (for hanging lights, cycle computers, etc), but in my opinion the biggest thing is that it can provide a solution to the thing I hate most about winter biking, which is rain. At least part of the reason I have pursued this work is that there were no good options for getting my hands on a nice fully faired HPV, and nothing spells freedom like being able to get where you want to go at any time.
The basic idea for how to build this thing is to decompose your ideal shape (for the example I use an elongated ellipsoid as per some of the photos in step 2) into crescent segments using a little math (don't worry, the attached spreadsheet does all that for you, at least for the simple shape used in this tutorial). Then you can cut sheet plastic (0.080" PETG or polycarbonate) into correctly-sized pieces and then tape the outsides of them together. If you're very careful about the outline of the segments you cut, you can generate a very good approximation to the large shape you want (similar to the way the almond-shaped segments of a football are stitched together to make that three dimensional shape). Once the segments are taped together, you can fill a turkey baster up with pipe glue and run a bead of ;pipe glue along the joint. You let it harden, clean it up with a Dremel tool, and use it! For the shape I'm building, I also show you how to attach it to your bike. This is accomplished by cutting PVC T-joints in half and then using pipe clamps to attach them to the bike. The mounts have proven to be kind of tricky to get right, however, and there's some room for improvement in this department.
The motivation for this work was that I wanted to be able to make big three dimensional shapes for a number of projects but was unhappy with the existing techniques. Either they required a big oven (necessary for most vacuum and blow forming techniques) or were messy and generated lots of extra stuff (ex: cutting foam molds for fiberglassing). If you still want to go those routes, the technique I show you here can be useful in building supporting structures for use in jigs and whatnot.
The one note that I should add is that for reasons I will detail in the wind powered bike instructable (I may also call it a "Side wind safe" bike) you have to be careful about the design of your fairing if you are using a front-steered only bike (whether upright or recumbent). Basically, large fairings are unstable in high winds (so you want to keep the cross section from the side low by adding openings or making it very compact) on a regular bike. In the wind powered bicycle project I show you how to build a bicycle that will get around these shortcomings (you have to steer the front and the back wheels), but you can also attach this to a tricycle (just keep the width of the vehicle as wide as possible and the center of mass low). The fairing I build in this example is a test piece and probably too large for everyday use (although you could try cutting holes in it).