I think I've found a way to whittle down the numbers a bit. it's not an end-all solution, but it looks like a good first-line defense.
Literally the dumbest way ever: Pit traps. filled with spikes.
Have a gap in your outer perimeter, with, for the sake of my tests, a 10 long, 2 wide, 2-3 block deep pit. line it with flagstone or whatever you like or leave it dirt. Fill it with spike traps.
I've been running tests with 25 arlenes and this has been the most effective, by far, of whittling down the numbers. Only a few jump over the pit, so you'll need to kill them. Some pyramid up. Sure, fine. But most stay in the pit where it's like shooting fish in a barrel. Bonus points if you put mines down at the bottom.
Most defense setups I've tried have left at least a dozen and more arlenes alive. This one left only 5, which is manageable from a defensive standpoint. This is also small enough to repeat at several points around a base, and the test was run with a doubled-door right next to the pit so no funky jump-overs are needed.
A 10x3x3 pit with 60 spikes and covered with loose floorboards was enough to kill all 25 arlenes as they tried to jump up onto unbroken trap plates. I'll consider that a win.
The 2:1 ratio for simple zombies is a bit concerning when you consider how beefy some of the other zombies are, and a cop explosion will, of course, clean the pit. By then, however, they still need to get out of the pit. Line with barbed wire and a fence so you don't accidentally fall in, and you're set.