This was one of my first Shapeways purchases. I'm commenting because I had a ton of questions/concerns/worries and figured if you ended up here you might have the same. (Full disclosure, and I hope @MagCAD
doesn't mind, the Selle Italia pictures above are of my setup.)
I run both a Cycliq Fly6 and Varia in the sincere hope I don't get rear ended on my bike. Frankly, I've told those I care about to check the camera in case the worst happens...sigh, the life of a cyclist.
My hope with this setup, which I have to say was met 100%, was to get the Varia up and away from my seat post so I could mount my Cycliq Fly6 in the space vacated by the Garmin. (I have decent space on my post but between the Varia, the Fly6 and a Tailfin pannier that I run when commuting, well, my post space on my Specialized CG-R was limited.
Back to the MagCad design and effectiveness. I have about 400 miles on the above setup and the Varia is rock solid in the mount and tucked up nicely underneath the seat. The mileage to date is a mixture of road and gravel...say 70%/30%. I'd say at this point, if it was going to work itself loose when riding, it probably would have done so by now. As for fit, once I turn it on, its basically unnoticeable as it does not rub or have any other fitment issues. Further regarding solidity, I actually don't remove the Varia when I transport the bike in a rack and again, rock solid is what I can report. Also in this configuration, it appears to be as visible here as mounted lower on the seat post as I had someone take a spin on my bike and I followed in a car.
I suspect it would be possible to mount a saddle bag and this at the same time but I happen to have mine mounted elsewhere on the bike (Wolftooth B-Rad system).