And probably the cheapest... all you need is some leftover fabric.
I had some leftover fabric from another project, so I ended up making this. :o)
It's a perfect beginner sewing project, as it requires only little sewing, and if you don't have a sewing machine, you can also handsew it! :o) (although sewing with machine makes it more durable, if you have a dog with destructive habits... :D)