We call them steamroller fenders thanks to their size and shape--they're 12" wide, 30" in diameter, with a 4" overhang.
The slick trick to these fenders is the flange. Typically, fiberglass fenders are made with the mounting flange as part of the fender--which is fine if there's a fender available in exactly the width you want, and you don't mind replacing a complete fender assembly if you hit a cone. On ours, you attach the flange to your car with bolts, rivets, or even plastic body panel hardware from your local auto parts house. Trim the bottom ends of the flange to fit the bottom of your chassis.
After the flange is mounted, you can slip the fender over the flange and flush to the body. Then affix the fender to the flange (rivets, bolts, hardware of choice...or if you want an invisible fit you can bond the fender to the flange with fiberglass and resin, or even epoxy glue) and you're done.
