I purchased the cover and pad that the Big Board company had.
The board is made from 1/2 inch very good plywood - the "padding" seems to be about 1/8 inch "feltish" polyester batting - and the "cover" is a duck or twill or canvas type cotton with elastic cording around it -
The cover is skimpy - it is washable, but has to be put back on when it is still fairly damp so I can stretch it to fit. I also have three strips of elastic pinned to the edges and going underneath the board to keep the cover it taut.
Being able to wash the cover is fairly important to me - gets rid of most of the water marks and scorch marks. (I use a roller to get most of the stuff off the cover, but every so often I think it needs to be freshened up)