I used fiber board with two reinforcements along the edges.
The fiber board has a smooth plastic surface, but I prefer to lay
a thick sheet of 30 mil POLYETHYLENE film as a release surface.
You can get the thick plastic from hastings plastics of Santa Monica, California.www.hastingsplastics.com
Mylar is also good for a really high gloss mirror finish.
Here are some photos of the rocker table I made:
It's great for controlling the rocker all the way through the board.
Works for directionals too.http://www.foilzone.com/phpBB2/viewtopic.php?t=10835
You can also order vacuum baggging stuff, inserts, and other stuff fromhttp://www.fiberglasshawaii.com/
Another good place to get materials from like ABS sheets fro rails and flex tips is a sign making supply place.
Just ask a local store who made their signs up front and call them to get their supplier.