Hi,
I recently made 2 pressing boards, to be used on a table, for ironing large areas. I found some really useful information on Sharon Schambers video:
http://www.youtube.com/watch?v=4LGbXou_u4c
She maintains that if your pressing board is too "cushy", it can make your seams "stretch" or "wing out". From experience, I have found this to be true. She uses OSB board, covered with one layer of warm and natural batting, and covered with 100% cotton canvas. I pre washed mine, and when it gets dirty, I simply take it off and wash it. (I made a pillow case type sleeve instead of using a staple gun to staple it to the board.) I have been extremely happy with the results. I hope this information is helpful to you.