I have a memory foam mattress, and I just flattened the box it came in and placed it on top of the slats so the mattress won't get deformed.  Someone else suggested getting pieces of plywood, but this is much more simple and cheap.

The quality of the bed is great, easy to assemble, and inexpensive.  The only problem I had was that it came in two boxes, and Wayfair did not notify me of the second box.  I came home to half a bed and had to call them to find the second one, which was actually scheduled to come a day later.  They should have supplied both Fedex tracking numbers.