MPH used cheap foam between the feedhorn and outer shell on most of their Python series K band antennas that has long since deteriorated with age. So the feed horn is able to move around with nothing to restrain it, plus there's a gap that allows any kind of crud or other foreign objects to get inside.

Will print with no supports. If you're using it on an antenna sitting in a hot car, I'd suggest printing it in ABS or PETG. PLA is going to get soft and warp. The outer shell of my antenna had a slight bell mouth opening so it might be slightly tight or loose (mine was on the tighter side) if yours isn't flared. If on the tight side, it should stay in place with friction alone.