I had to build a hovercraft for a Science Olympiad build event.  It had to work off 9V max.  It had to lift 5 lbs for max points and travel a 6ft distance as close to 15 sec as possible.

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I researched a bunch of fans and motors with help from my dad, and found a powerful computer cooling fan for the lift, and a hobby motor for propulsion.

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Designing the base with the right way to trap air and create lift took a lot of experimentation.  In the end, I had to use screws to prevent the plastic bag cover at the bottom from expanding out and creating too much friction with the floor.  This seems to give good hover ability and also let it lift 5 lbs easily.  I used penny rolls (16) for the 5 lbs weight as it was in the competition.

 

Since the craft had to travel in a wider channel than it's own width, I found that it would keep rotating and hit one of the walls, which would cause more friction induced turning. So I came up with the idea of putting in shock absorbing rollers on the sides that would help it grab one of the rails and not turn even more from friction.

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I also researched propellor sizes and number of blades for optimum performance for the given constraints and ran many detailed experiments to get the best configuration of propellor diameter, number of blades, and operating voltage for the motor.