Sorry for the late response, but I wanted to drop in here to help.
I understand that you're needing a replacement and I wanted to stop by and see how I could help.
Just wanted to double check to see if you were still needing help on this? If so, I would suggest reaching out to customer support.
Please let me know if you have any other questions.
Just found a great workaround to get a new backplate if Google won't sell/send you a replacement. I found this cad file online for the backplate (https://www.thingiverse.com/thing:3328392) which is the file you'll need to 3D print it. I didn't have access to a 3D printer but found this marketplace (https://www.treatstock.com/) for those who do. It's like Airbnb for 3D printers! I found a reputable printer in my area and uploaded the STI file to them for a quote and got them to print and mail it to me for less than $15. ($10 to print, about $5 to ship). Worked like a charm. See attached picture. The one on the right is the 3D-printed one. The one on the left was the one Google eventually sent me after I DM'ed them on Twitter. I actually am using the 3D printed one (which works fine) and gave the new spare to a friend also in need of a new backplate. Hope this helps!