I would recommend making the panniers from leather, use a good stiff 12oz or so leather to keep a good stiff side, and use metal wire for anyshaping. If you use heavy weight leather, they would be plenty durable enough for trapping. I wouldn't recommend wood... just for comfort of the horse. The metal will have a little give and conform to the shape of the horse. Also.. the size to strength ratio of the wood might make it take up space inside your panniers.
After you have made your leather with the frame of shape inside with a heavy gauge wire... then you can take your deer and contact cement it onto the leather. that would make it even more ridgid... heck - I bet you probably wouldnt even need any type of frame if you started with good heavyweight leather and added the deer as a good look.
