Anne Finch's method: The Lazy Way (not having to use bias binding)
Using the template below as a pattern (each square on the grid is 6" (15 cm) sq.), present the top edge of the template to a fold in your material and cut it out double. Do this twice, and with right sides together, sew all round the outside edges leaving a small gap to be able to turn the whole thing inside out. Sew a six foot band of soft material at least two inches wide onto the middle of the back for the belly band. Finally, oversew or bind the two thick edges which go over the breastbone, very firmly together. This can be a weak spot.
Standard blanket/coat material can be used double unless it is particularly thick and could do singly, perhaps with a flannelette/soft cotton lining. Another idea is to cut up and use material from second hand padded anoraks (which have the advantage of being waterproof,) or padded dressing gown material.
We'd like to extend a special thank you to all those knitters and seamstresses who make and send coats to us to send to Spain or for use by our dogs in kennels here. Especially we would like to mention our special friend Katie in Cornwall who sends two 15 kg dog-food bags of coats per week and hides Mars bars, bath salts and relaxation tapes amongst the contents to make sure Carolyn and I open every single one and keep up our sugar and hygiene levels and our sanity in this office!
The alternative coat patterns (and from the USA!)