If it were me (and it might be in due time), following George's suggestion for slow rotation I would see if I could find an old Raytheon Pathfinder 2500 or 2600 marine radar scan head like I used to have on my boat, or the Furuno equivalent from the early 80's. The scan head has plenty of real estate in it once you remove the magnetron and related electronics, and the drive motor could be replaced with something much slower or driven at lower voltage (they had 12 & 24vdc models). The rotating antenna is fed via central waveguide, meaning it simply rotates around the fixed waveguide, and you could just put your dongle below and let the motor spin the head around above it. I seem to recall they were s-band rigs (2-4 gHz) but I think the gain and directivity of the array would probably more than offset any wavelength dependent characteristics. Mine was a 2600, which had 50mi range, primarily a function of transmit power, but as a receiver it should easily get those ADS signals out to the radio horizon. Could probably get one of them out of a dumpster at a marina or from some old codger who repairs antique marine electronics for free. If neighbors ask about that thing rotating on the top of your house, just tell them you might need to move at night. Of course you could always do the same thing with a domed unit because they probably have a similar internal setup. The great thing is that it's already weatherproof and could contain everything you need including the PI.
Just another odd-ball project idea served up for you from a certifiable oddball.