Sheets for Chicco Lullaby LX Playard??

Can anyone recommend sheets that fit this play yard well? I've seen a lot of online discussion on what DOESN'T fit, but not much on what does.

Also, do you generally put something waterproof under (or on top of) the sheet, even if the playard mattress claims to be waterproof, that way you don't have to wash stuff in the middle of the night if there's an accident? And if so, is that safe, or is there a danger of it bunching up and causing suffocation? Sorry if that's a silly question, FTM here. :)