I use the blue camping pad foam for the edges, ($5 at any Wal Mart) though I would be interested to try something new. The swords I make hold together real well largely because of the material I use for the flat though, which is from ACOR orthopaedic. The brand name is "microcell puff", I get it in 1/8" thick sheets about 3' x 2' for something like $10 IIRC.
They also have a variety of other foams but this one seems to be the best I could find for this purpose.
You can reach them at 800-2372267
their website is
www.acor.com Good luck!
Jeanry