Here's my two cents. If you make them with a part that extends up between the two wing shoulders on the back, they will stay in place a little better, won't slide side to side as much, and the elastic will fit around the wings better. It looks like maybe you have one elastic running around the body? It will stay on if you have two loops of elastic, one on each side that loops over the wing and rests around the shoulder joint. The wing slips through the loop and rests on top of the saddle, in a natural position and free to move. You can see the attachement points on the picture below, in red. I also attached a picture of one of mine (that needs new elastic) so you can see. For my standard sized birds a 5 inch piece of elastic, which ends up about 4.5 inches is about the right length, I have a few birds that a bit shorter works better, but you do need room to put the wing through. You may have to adjust the length to make them fit your particular birds.
Lastly, if you make them out of a tightly woven fabric (I use indoor/outdoor fabric like sunbrella as it sheds water well-but something like denim or canvas works well too) they will be less likely to hang up on pin feathers or broken shafts and cause irritation. Two layers of fabric with a bit of thin batting or fleece between them makes them a bit more substantial and helps them stay in place. And lastly, they don't have to be beautiful, just functional and get the job done. Mine last for years and years, I wash and dry them, I only have to replace the elastic when it gets stretched and worn. The one in the picture is probably 8 years old. Hope that helps some.
Lastly, if you make them out of a tightly woven fabric (I use indoor/outdoor fabric like sunbrella as it sheds water well-but something like denim or canvas works well too) they will be less likely to hang up on pin feathers or broken shafts and cause irritation. Two layers of fabric with a bit of thin batting or fleece between them makes them a bit more substantial and helps them stay in place. And lastly, they don't have to be beautiful, just functional and get the job done. Mine last for years and years, I wash and dry them, I only have to replace the elastic when it gets stretched and worn. The one in the picture is probably 8 years old. Hope that helps some.
