There is no requirement that the antenna be oriented vertically. It will work just as well, and maybe better if horizontal. What is definitely a good plan is to orient both the sending and receiving antennas the same. Signal strength looks like a doughnut, so if both antennas are oriented the same, then the "fat" part of the doughnuts play well together.
The main reason a vertical orientation is used is to get a part of the antenna over the top of anything between the sending and receiving antennas.
FWIW, I have both of my antennas horizontal and get good results. Additionally, I actually hang my receiver upside down. I use some women's hair bungees to attach it to my trucks rear view mirror. Right side up, there wasn't room for the antenna. I set the monitor to flip the display so I see the image correctly. Only issue is the "distance guide" that shows on the display is wrong (it doesn't flip)...
I am starting to think, that I will never be old enough--------to know better.
Tolerance will reach such a level that intelligent people will be banned from thinking so as not to offend the imbeciles. Fyodor Mikhailovich Dostoevsky, Russian Novelist
S.E. Mich. Flagstaff 26FKWS / 2022 F-150 3.5 EcoBoost SCrew Propride