Rupes 21 backing plate options

Was wondering if there are any other backing plate options for the Rupes 21 out there?

I have both the 6" and 5" (from the 15) that I use but was wondering if there is an even smaller backing plate available for the 21.

 

there are no other backing plates for the LHR21.

you can consider picking up the LHR75.

 

This is what I have plans of doing, however I was hoping to find something that would work ASAP to get me there.
I will have to order up a LHR75 from you guys in the next day or so.

Thanks!