Update:
I have made twin fans for the hovercraft, which I will install shortly. They took about four hours to build (without curing time for the epoxy) and I am fairly happy with the amount of air that they move. Hopefully they will be able to inflate the skirt and lift the craft - if not, then I will end up with a normal prop doing the lifting and two overly complicated cooling fans! (So lets just hope they work!)

The centrifugal fans are modelled off the ones used in real hovercraft and can hold more backpressure than a normal propeller. They are two inches in diameter and 3/4" tall. I started with a 1/8" thick piece of oak which I cut into circles with my scroll saw. Then I cut the blades out of 1/32" aircraft grade plywood which I glued in with CA along the angled lines I marked previously. With that done, I sanded the blades so they would be of equal height and glued the top "doughnut" on. Then I drilled a hole in the center and press-fitted the fans onto the shaft of the 380 motors. I reinforced this connection with CA and epoxy. Then I ran the fans on a 3.6v power supply against a block of sandpaper to ballance the fans.

To make the shrouds, I wrapped a piece of 1/32 plywood around a 2" pvc pipe and glued the edges together. This formed perfect ducts for the lift fans. I capped them with balsa "doughnuts" and let the epoxy cure. Then, using a dremel tool with a diamond cutting wheel, I cut out slots which were then surrounded with 1/8" pine cutouts. These were rounded and covered with thin plywood, creating channels for the airflow. This will be ducted into the skirts. Lastly, I attached a semicircular backplate and placed the fan inside the shroud. When this had been glued, I attached sturdy motor mounts which secured the motors while allowing me to adjust the distances between the shroud and the fan. For all of this work, I really hope these fans will work - I will be very dissapointed if they don't.