To put the camera on the car roof I currently use a triple suction cup mount with 9 cm diameter cups. Once in a while one of the cup won’t stick when I put it in place. So be sure to verify that each suction cup sticks after putting the camera in place. Also I have a bottle of water and cloth so I can clean the roof before putting the camera in place (I also use it to clean the windshield since I also take front-facing pictures with my smartphone).
In any case I never found more than one non-sticky cup even when driving on uneven small roads (90 km/h) or on expressways (130 km/h) in winter. Plus everything is secured via a secondary cord that’s attached to the hand grips of the rear seats inside the car.
Note that suction cups don’t like cold very much but I still had success taking pictures in december and january when the temperature was below 10°C. Anyway, when it gets really cold the other problem you’ll face is frost on the camera lens.
Up the suction cups are a couple of aluminum GoPro extension arms. The aluminum arms are much stiffer than the plastic ones so I like them better.
And finally for the camera I use a KeyMission mount and a silicon cover so I can thread a cable to (hopefully) secure the camera in case the rest breaks off. Regarding the 1/4" mount, make sure to use the KeyMission one (just cut the pin that sticks out). The little round ones won’t reliably remain screwed tight.
For high-speed runs make sure the camera faces sideways: this minimizes wind drag, prevents the articulated extension arms from just folding, and almost entirely avoids bug splatter on the lens. In fact, I point the Mi Sphere to the left so that when I step out of the car I can see the leds blinking and thus easily check that the camera is still taking pictures in interval mode.
Here are the parts I use so you can get a better idea of what I’m talking about:
Triple suction cup mount:
Aluminum extension arms:
Xiaomi Mi Sphere silicon cover:
Fish line and ties to secure it all (selected for its thickness so it’s not too sharp):
Add some GoPro screws from wherever.
Throw in an additional 10000 mAh USB battery in a mesh bag attached to the suction cup mount, a 128 MB microSD card, and you’re good for 8+ hour drives.