Outback pannier rack use 36 mm long spacers. Our Top luggage rack use 29 mm high spacers and thickness of the stainless steel sheet metal bracket is 4 mm, which gives combined length of 33 mm.
Outback pannier rack can be installed on top of Perun moto Top luggage rack. One M8 large washer should be installed between Perun moto Top luggage rack and Outback pannier rack (on all four contact points, of course). Then bolts that come with OM rack can be used to tighten the assembly, as thread engagement is within 1 mm range of the thread engagement designed for OM racks.
To summarize - install Perun moto Top luggage rack first. Remove the bolts that came with Perun moto rack (and 3D washers). Place the Outback pannier rack above Perun moto Top luggage rack side brackets and put M8 large washers between. Then use bolts and washers that came with Outback rack and tighten everything.
M8 large washers are standard hardware, and they can be found in any hardware store. Of course, for customers ordering from us, we can add few washers in the package (just email us before we ship the order).