Years ago I looked into Prometal’s 3D metal printing process. The results were impressive but the stainless steel is magnetic:
This disqualified them as magnet holders!
Recently Prometal let me know about an experimental bronze process they are developing. Bronze is totally non-magnetic. BINGO!
Prometal donated a test part to see if I could make an airtight braze with a TIG welder:
The part was to be brazed to a Swagelok VCR fitting like this:
Prometal is still adjusting the process. This test part is a bit oversize so it was difficult to braze, but we got something:
The bronze attaches to the stainless nicely.
I was excited to put it in the chamber like this:
But I screwed up. I welded on the wrong VCR cajon. This one is two short to catch the male thread once the gasket is added.
Dammit.
I had the right part here all along; shown on the left:
So no vaccum check for now. But it looks like the 3D printed bronze parts braze nicely.
I count that as a win. Think of the parts I can make with 3D printed bronze + brazing.
Prometheus Fusion Perfection 
did you see this?
http://nextbigfuture.com/2011/11/emc2-inertial-electrostatic-fusion.html
Sophos tells me that the link to Prometal’s website you posted is a malware attack site. Are you sure it’s the correct URL?
could you swap the gasket for a small Cu washer?
Perhaps even a small piece of foil might seal well enough to work – if it does hold vacuum, this might be good enough to tell.
If it doesn’t, you’ve only wasted a little time, since you won’t know if the leak was in the part under test, or the fitting, in which case you’ll need to cut it up and try again.