Miriam, my son helped me make a large ironing table. Used 1/2" plywood, covered with many layers of thick cotton batting/felt (whatever it was, it was cheap). Wrapped the cotton under the wood and stapled in place, then used safety pins to attach unbleached muslin on top to use as a ironing surface. Have not had any issues with moisture or mold, etc. I use steam on everything.
You could make a layered pad the size you need, attach some string ties to it and tie it to the table to secure it. That way you could remove it when done. Although I like the marine snap idea.