Attys don't sit flush on the Duke either, small gap as the 510 is just ever so slightly raised. Corners on the cover are rare earth magnets which hold it snug, simple adjustment wheel on top to turn voltage up or down. Build is, from what I can see, identical between the Duke and the Hex which is why I suggested going to the origonal maker and ECFer (Goss) for the Duke if you could.
I see you are not bringing the DNA30 devices into the discussion so that opens an entire new line of debate. The Duke isn't flashy, no bells or whistles, it's just the best street fighter on the market IMO. The DNA30 devices I've seen are much more sexy but come with a much higher price tag, except for the clones I guess.
Let me put it to you this way... I know many that have purchased DNA30 devices, used them, sang their praises for a while, then put them on the classies. I have only ever met one person that sold their Duke and they were very very very sorry that they did.