Just did exactly this about 6-months ago. Checked around and got a plastic-coated pad – was really advertised for it’s stain-resistant quality, but also useful for this application. The plastic-coated side (really, it’s more like a thin layer of rubber) goes on the carpet-side, and should anyone spill something, won’t absorb into the pad. The cost wasn’t significantly more than the regular pad, and the installer said he’s seen these a lot, especially in basements.