I went to Ikea and bought 3 euro single duvet that can be easily washed over and over. You can basically use any duvet but I don't recommend ones filled with feathers.

My dog gave me a hard time when I was taking these pictures. He just wanted to play. He's only 8 months old so everything is exciting.

Then I found an old cover that I didn't use anymore and put it over the duvet.
Any cover that fits the duvet can be used. I just like this camo cover.

And then just fold it in two and you have a warm and cosy bed for your dog!