I thought I'd follow up after having a couple of weeks with my new light mount.
The first observation is that the Cygolite side is too high, should have been offset 23 mm or more. As it is it's too high to put a flat load on the top of the rack.
Second observation is that the Cycliq mount is loose. My light mounts tightly in a Cycliq mount, but in the 3d printed one it has room to rotate (on the axis it's twisted in) as well as wiggle vertically and horizontally. I do have some thoughts on what's going on here.
Third observation is that having mounting holes that use standard socket head cap screws is awesome. I've used all sorts of mounts and nothing beats being able to tighten them with a normal hex key. For example, the Cygolite mount uses hex head bolts in hex-shaped recesses so you have to awkwardly deal with tightening a nut on the back. The Cycliq mount uses flat head machine screws that are an odd size and end up rusting.
So some good and some not so good, but fun to try regardless.