It's very unlikely there will be an 18xxx device that is the same diameter as the 19mm tanks. 18xxx batteries are generally 18mm in diameter, which means the device would have to have a wall thickness of less than half a millimeter and that doesn't include the additional thickness needed to machine threads into the tube. In short, it won't happen.
A 16xxx powered device could most definitely be made to fit a 19mm tank, but AFAIK such a thin 16xxx powered device doesn't exist.
You might have better luck getting the larger diameter tanks and finding something that will fit that.
I know exactly where you are coming from. Diameters that randomly change at different sections on the device, things sticking out, lack of symmetry, these things bug the hell out of me when it comes to PVs. I made a thread in the eGo section a few days ago trying to find specific eGo batteries, because the majority of ego batteries have a smaller diameter ring where the button is located. I wanted the ego batteries (Rivas I guess) where the ego battery is 100% the same exact diameter the entire way up to the 510 connection.
I wish more tanks were made to fit specific devices. I love how the ZAP has the skirt that fits seamlessly with the Provari. It would be cool to have something that had skirting to fit a zmax, or that was the same diameter as a Vamo. I'm getting so incredibly picky about form these days that I honestly don't see myself purchasing anymore new hardware until I find something that doesn't upset my OCD.