I recently built a mount for a small Pelican style case to be mounted to the rear side glass of my SUV (if you search GrizzlyPath on YouTube you'll find a video I did).
Regarding aftermarket and/or DIY suction cup mounts, I did the math and the force applied to the window is extremely minimal. Especially the more vertical your window is. Whatever you have it mounted to on the roof is what is going to bear most of the load. In this instance, my diesel heater in the case is approximately 30-lbs.
I live in a heavily wooded area and based on what I've seen I wouldn't worry too much about it being ripped off. One would want to be cognizant about it, but it's pretty dang sturdy. Obviously depends on how you build it.
You didn't ask, but in this instance I wanted a solution outside of keeping it on my roof (when no roof space is available) or in a hitch cargo carrier. Additionally, I didn't want to keep it in the vehicle because of the diesel.
I should also add that I used easy to remove knobs so the thing comes off in seconds which is great.