I have been using a duck harness for 4 weeks now. Bob is 6 weeks old. I have used regular diapers cut up. The Goose's Mother recommended a size 5 diaper (I think). The piece they sent me to show how to cut it seemed very large to fit in the pouch and it stuck out all over which caused Bob to pull at it! I wound up cutting it smaller and it works great! I also have used thin, sanitary pads because they have adhesive backing. I can get 3 "inserts" out of the overnight, long pads. I didn't think of using puppy pads, but they would probably work too. I have found though that the harness does get wet on the outside even though it looks like there is only poo in the diaper.
Good luck and have fun keeping your friend inside! BTW, where did you purchase your harness from?