Kwik Tek STIK-a-FLAG Water Ski Flag
academy sports was clearing these out for a buck a year or so ago (they still carry them at the $7 buck price though) and i liked the look of the suction cup.
Kwik Tek STIK-a-FLAG Water Ski Flag | Academy
long story short, best suction cup ever and the center hole for the flag pole accommodates a F to BNC female + F coupler combo.
RS800 on bnc/top side, 3ft rg6 coming out the bottom with F to BNC male adapter and slapped in right rear window. never has it come unstuck (held up under multiple jerks, me included) and the best solution i have ever managed (there have been many tried).
yeah 7 bucks for a suction cup is horrible, but if you can force yourself, i think you'll be pleased.