Here is my solution for mounting a Varia RTL to a saddle pack.  I selected a Topeak Aero Wedge Pack (medium size) because the back flap is fairly flat and vertical.  There are certainly other packs that would work as well.  After piercing two screw holes through the back flap of the pack, I mounted a Tail Light Seat Bow Mount sold by Sheawa on Amazon.  This is meant to be mounted on the saddle rails, but it works as shown in the photo below just as well.  There are other similar mounts available.  Although not necessary, inside the pack I added a piece from a corrugated plastic lawn sign to add rigidity.