I don't know if they make a unit specifically for the top of the washer/dryer but places like the Container Store and Storables have many different plastic baskets and bins that would work. I use a plastic basket from Target. It has a solid bottom to catch leaks. If you want something more decorative a plastic-lined wicker-type basket looks good.

You can buy a work surface at sears, or any whirlpool dealer. It's a solid suface and fits over both machines (if they sit side by side). It is lined with a rubber mat for easy cleaning.

Not too long ago someone posted that they use the white resin folding table on top of their FL washer and dryer -- that it was just the right size and provided a cheap top.

My own experience with a flat top for storage of detergent and other laundry items is that they will move around and fall off or fall over when the washer and dryer are running. I guess it depends on the washer and how much vibration you have, but I like things corralled in a basket. I have even taped the basket down in the past, but that is not necessary with my current washer. If the topper is suspended off the washer it should not be a problem. My friend has her large shelf hanging from the wall with cables so it can be lifted up for service.