The AS&E article has instructions for putting him on a open-necked top and yesterday I managed to find a suitable one at Cotton On Kids. I was in two minds as to whether to follow the instructions and cut a hole in the top, or just try to create and add a patch. In the past I have always avoided actually cutting a ready-made garment. It feels like vandalism.
In the end I took the plunge and cut.
Fortunately, it worked quite well. It was hard to stay perfectly straight, as the fabric has so much give in it, but it is also manipulable.
The end result looks quite good and seems strong and durable.