Here is a link to the free tutorial of which I based my own nap mat. It has an attached blanket which I think is a great idea! However, I chose not to attach a matching blanket due to cubby space issues.
To keep the nap mat inside the cover, I used a strip of velcro just inside the hem.
Or, you can always buy one on Etsy! There are several different sellers and they make them with or without pillows and blankets attached.
Here is the final product: