I used one of those for some time. This comes with a rubber to cover the gap, a rubber beading. Not very comfortable to put on. It is useless without that rubber strip because the car needs to be completely sealed to exhaust hot air out of it, otherwise air from the gap will circulate in and it'll be a problem from rain and dust coming in.
It works for me, used for my old car, still resting in the glove compartment but hardly use it. this thing needs direct sunlight otherwise no fan motion, need to be careful which side the sun will be, and wont work with the tinted glass. It cost me around 1500/- year ago.