In the early days of personal hovercraft manufacture fiber glass was used extensively to build hulls in just one piece. For one thing, the material is very light and so the power to weight ratio is well taken care of. Of course, bigger craft have by necessity hulls made of stronger and more rigid materials, such as aluminium or composite polyester construction, but the particular design characteristics of personal air cushioned craft for leisure use means that certain plastic scan be used. The huge disadvantage of GRP is that it split when hit with a hammer, or when it comes across an obstacle in it's path. It's expensive to repair and takes time.
Many manufacturers are turning to new materials that are in tune with the small hovercraft requirements, namely, lightness, robust and maintenance free. High density expanded plastic delivers all of the above and more - see diy hovercraft kits. As an added bonus, it can be supplied in any color that's requested - the dye is simply added to the plastic granules before the hull is cast, so there's no paint to upkeep and worry about. Hence, it is maintenance free. Most personal vehicles are less than four metres long, so it's possible to mold the hull in one piece, which saves a lot of time in the manufacturing process. Holding bays for engines and other important fitments can be designed right in, without having to worry about fabricating extra pieces later.
One manufacturer of leisure hovercraft present demonstrations where he repeatedly bashes one of the company's hulls with a ball pein hammer. Of course, the ball of the hammer dents the plastic and if were mine I wouldn't be too pleased, but the point is there is no splitting and no extensive damage beyond the point of impact. No expensive repairs are needed or even necessary, which represents a big advantage for prospective buyers and family men concerned about the safety of their kids, for example.
The concept of monocoque construction lends itself beautifully to hovercraft manufacture. All the holes for steering linkage and engine housing are simply factored in and the steel mold is machined ready for production. Plastic molds are expensive, and are intended for runs of many finished parts, not just a one off, unless it's a prototype. Even cupboards and other accessible areas will be present in the finished article. As the plastic expanded material has the consistency of wood, fitments can be fixed using special marine screws.