I used 7/16" OSB plywood, one layer of all-cotton batting and heavy canvas as the cover. I ground the sharp points off the corners with my mouse sander and I soaked the canvas and let it dry after I had it stapled to the board, to shrink it up tight.
I LOVE the hard surface, with just the slightest bit of give. It really seems to to help get sharp folds to have so little batting under the canvas.
Next time, I will cover the top of the wood with aluminum foil - it will deflect more heat back to the item I'm ironing and it will prevent the steam/moisture from penetrating the wood.
I will also make a pillowcase-type cover for it of cotton percale, so I that I can take it off and wash it. The canvas is becoming really starchy and dirty after two or three years of use.