okay, skirts... lets see...

Materials - anything that is relatively tough, waterproof, and holds air... for mine I used fabric from an old pair of athletic pants...
Cheap - use a heavy duty trash bag... you know the contractor bags?
Shape - while finger and bag is nice, it is complicated... just do a simple bag design, get the plans from the website I linked previously.
Inflation - I have tried almost everything short of a miniature jet... the best was a normal prop, about 7" in diameter in a close fitting duct

After seeing your design, base the bottom half of your build off of griffon 2000 plans so you can use the skirt layouts. However, instead of using plywood and balsa, use 2"-3" insulation foam board... the pink stuff. That saves a lot of money and is easy to work with. carve that to the general shape of the plans, should look like this:
___
( ___ ) from the top view. Then you cut out a hole in the front of the slab for your lift duct and route two canals for feeding the skirt. Put the skirt togethter with duct or gorilla tape, and then tape it to the sides of the slab... you have a hovercraft. Now just make the deck whatever you wish... an LCAC in your case.