stainless steel. Basically a frame would need to be built around the washer & dryer by using end panels to support the countertop. Keep in mind the top will be higher than a typical countertop.
stainless steel disaster pan under the washer dryer in case a hose or the washer ever fails. If on a second floor a flood can do a lot of damage to room below.
It wasn't quite that easy in reality because they barely fit in the closet and there is a stainless steel disaster pan under that would stop