I just went through this myself. I think they are having trouble with their site right now. If you try to Search it says the Catalog is temporarily unavailable.
Their part number is AJR 611 211 Battery Adapter Plate. It cost $30.97 plus $9.00 shipping. It's pretty basic but it works. Here is a pic from someone else's post:
I recommend you file the pointy angles to soft curves so they don't scratch anything. Here is my Optima 34R installed in my Slantnose.
