Following a comment for one of the Rear Rack owners, we decided to test an additional spacer, between the tail and the Rear Rack, similar to the one used with the standard Rear Rack.
Actually it is not necessary, as the Rear Rack HD uses 4 screws, so it is stable as it is. But the additional spacer increase the stability of the Rear Rack, making negligible the (unavoidable) already small deflection when loaded.
The spacer also provides an additional attachment point (personally I have used to secure a small tool bag positioned under the rack).
It needs a conical washer and a countersunk M8x80, but a standard M8x75 with standard washer works as well.
The spacer is available for free on Thingiverse (search our name, the spacer is named FGWX-0-B-003).