Contacted Customer Service through e-mail and got a quick response. Ordering was easy through website.
Back panel - right for: ada/asa/aua51 we
Back panel - right for: ada/asa/aua51 wet section (18 1/4 x 27 21/32) wh9al
Buy this part and as many parts as you like and pay only $9.99 shipping, within the Contiguous United States.