when, I ordered some other parts, I had their support add in a back plate. Shipping costs being what they are, it is good to get some of these items when you order larger itens to minimize shipping impacts.
just contact purism support. they ask for the order # and also ask you to put a note in the order instructions so they don’t ship the items without adding in the backplate.
worked well for me. I have a spare backplate now 