Hi, What are you trying to achieve - Hide the appliances?
a thick piece of ply wood works over the tops. you would need padding across the machines to keep them scrathing.
What might be easier that trying to build your own folding space is to simply purchase a length of laminate countertop and mount it over the two appliances, wall-to-wall if possible. You will instantly have a waterproof, durable, easy to clean, finished folding surface.
You could build a "table" that frames each side of the appliances just above the tops. No scratches or rubs and removable if cleaning or servicing is needed.
We used simple 2 x 4's on ours nailed together and stained. http://alifeunfolding.com/blog-1/2017/11/13/small-space-big-impact-the-one-room-challenge-reveal?rq=laundry%20room
We used stained 2 x 4 here. https://www.alifeunfolding.com/blog-1/2017/11/13/small-space-big-impact-the-one-room-challenge-reveal?rq=Laundry