A small amount of spit for a seal and do it while the air is at low
pressure eg while as high up as possible.

Well, seems this works fine, I have just tried this technique on a 3.5 hours
flight up to 7000 feet.
Suction mount fell off after 30 minutes of flight (after being attached on
the ground), but it didn't fell off after being attached on 7000 feet!
I'll try water instead of spit next time.