I just use a decent quality inexpensive fitted sheet from Target or any home store. Full size fits most hard shell tops well, surely it won't be as form fitting as one made for the odd RTT mattress size but they tuck under ok. It works well in the Maggiolina at least. Throw a sleeping bag or such over the top and you'll never notice. I keep a clean one available in the house and just rotate them out every few trips.
Also, i did add a thin mattress pad cover and then did the fitted sheet over the top but I found the thin pad added just a bit too much height and wouldn't allow the top to close properly so that idea was scrapped.