In the meanwhile I found that it is quite impractical to fix the mounting brackets to the mast top. Actually it seems like this antenna wants to be mounted on a reling or a shorter mast.
I had ordered the antenna together with an A026+ for maximum AIS range. It only makes sense to mount the antenna on the mast top for this purpose.
Do I have the right antenna or should I have bought as simpler steel antenna with a proper foot that I can easily attach to the horizontal plate on the top of my mast?