It's not ideal and luckily we don't have to do it that often but if you cut off the two metal clips that hold the horn to the mount you can reverse the bracket. Everything lines up and the two screws in the front hold the horn/radio in adequately without the clips... at least it feels secure and it's not like there is much/any force trying to pull the horn out of the dish when it's mounted.
Interestingly enough it appears that someone at some point in the design actually intended for it to be reversible. The slot/hole in the side of the mount actually lines up with the LED's when reversed, everything appears it was intended to work that way but somewhere along the line someone messed up. The only thing keeping you from reversing the dish is the slots for those metal clips are off by just a mm or so otherwise it would reverse just fine.