I have no pattern, but I have made them, is ridiculously easy.
Measure dog from shoulder to floor, then from chest to arse.
Add 30cm in each direction for a medium sized dog, probably a little less/more for a very small/large dog.
Cut out two pieces of rectangular towelling in these measurements.
Lay them down and cut off a corner where the dogs head will go.
Sew together on three sides.
Attach strong (wider and stronger than you think you will need) velcro to the other side.
And Bob's your mums brother.
Alternatively, seeing as good towelling is not that cheap, buy one from here I have this very one, it is 15 years old and still look brand new.