Depends on how silly you want to get and how much space you want to give up. Tapped horns using 15" or at 18" drivers for example will take up a bit of space, but get pretty loud if you give them enough power.
4.5 cu.ft.? That's about the gross size of my POC3 TH. But that uses 1 12" driver and 1200 rms will be way too much for it.
If you're look for decent outdoor SPL, two pro audio 12" drivers (that are suitable for this purpose) in a ~40 Hz vented or MLTL box that size should give some pretty decent output with 1.2 kW (hopefully that's into 4 ohms). Interestingly enough, I was just talking about that in a FB group - see https://www.facebook.com/groups/bassaz/ ... 590724736/