The blanket doesn’t fit because the neck is too large. Does it have enough length along the spine to cover the rump if you place it forward of the wither to where you’d like it to sit? If it does then all it needs is an extra chest strap placed high enough to keep it there. If there’s not enough length along the spine then there’s nothing you can do to get it to fit the way you want. You need to lay the blanket forward by a few inches and add a new chest strap high enough to keep the blanket in this place, but low enough to not cause too much pressure when your horse puts his head down. Blankets without nylon linings don’t slide on the coat easily and will always pull back as far as the neckline will allow them to, so getting the placement of chest straps correct is really important.
Chest extenders only work if, when the blanket is placed forward of the withers, there’s not enough length to the chest straps,or coverage at the chest. That’s the opposite of your problem. When you place your blanket forward of the withers you’ll see that you have too much chest room.
Using these plus some easy to sew through webbing are the simplest way to add a chest strap:
You can sometimes get away with just adding some darts in the neckline when it’s too large, but given how much shoulder is pushing out and how much further forward it needs to sit, I suspect that adding a chest strap would be easier.
