should be easy enough to make your own, with proper tools.
tap a rod of SS with a 7mm x 0.5 tap.
get some type of plastic rod that you can force into the threads you just tapped. Delrin is a good choice for this.
drill and tap the centre of the plastic rod (+/-3mm dia.) and thread a screw into it.
now you have a positive and negative for a 510...
and because the centre post is threaded in, you can adjust it as you please to your atty/
tank/carto.
and if your box mod is made of metal, you can just tap into it, and use the case as the negative of the connector.
its easy for me to type this out, but i've tried, and got really frustrated... i just bought a mod, and called it a day.