Let me get my kids to help me post pictures tomorrow.
I make them 12 inches square...with corners that stand up out of heavy muslin or duck fabric. I zig zag every seam since I don't have a serger, so that nothing ravels.
Then I make the covers out of flannel with a flap for a closure so that the cover can be washed---some are cutsy kid flannel...some are guy flannel, some and just normal flannel print.