There is no 'backing' as such.
Basically I have made an MDF surround which, with the leather in between, fits the monitor snuggly.
The original inner plastic plate had to be trimmed a bit and the way the metal tube fastens onto it also had to be modified slightly, and then it was a matter of cutting out excess foam.
I do have pictures of the process and will be doing a tutorial at some point.
There is already a tutorial by Cris who made headrests from scratch, well worth a look
Fabricating new headrests