I use the pish pads cut to size to fit the whelping box. They are sort of felted on the top and absorbant, but backed with a kind of plasticy stuff so they stick to the plastic box kind of non-slip and are almost impossible for the moms to dig at(mine don't even try anymore). You can stack a few on top of each other and it's easy clean up when you just pull the soiled one off the top and there is a fresh one right there. Machine washable....love them.
www.pishpad.com developed by Maltese breeders near here.