Too bad the ZAP turned you off of the push-on type caps. It is infamous for being too loose.
I have a new model AGA-T and, mounted on a heavy, stainless, 18650 mechanical mod, I can shake it by the cap and it won't come off.
Same here. Mounted on my ProVari with the 18650 battery in there, the top cap's just nice and firm.
Hell, I pulled it off with such fervor last night, the top cap flew across the room.
I'm partial towards the push-on caps because I can alight the air hole with the wick, or where ever I want it without sanding or modding the top cap...but I guess that's almost going against the nature of the modder's section, huh?
In any case, though, it's worth mentioning that my friend's AGA-T+ rev. 2's top cap isn't quite as firm. We've remedied that by using a fatter o-ring, in combination with some sewing thread wraps beneath the o-ring. Fits like a glove. I hear you can use dental floss as well.
If you decide to give it a try, best of luck! If it doesn't work out, or you don't like it for whatever reason, I'm sure you'll be able to stick it on the classifieds without taking much of a loss.