The parts from ProMetal just arrived. These parts feel heavy, dense, and strong. You really have to hold these in your hands to believe it. They hold water without leaking. Cost ~$30 each. 
There is some texture, but overall these parts are highly conformal to the design. 
This is very exciting. The first prototype core is within reach.
Next I will grind the touching surfaces of the lids and try to laser weld them together.
On the down side this material is magnetic:
I wonder if ProMetal can adjust the composition of the alloy. Stainless steel can be either magnetic or non magnetic depending on the alloy:
There are different types of stainless steels: when nickel is added, for instance, the austenite structure of iron is stabilized. This crystal structure makes such steels non-magnetic and less brittle at low temperatures.
Update: Looking at the ProMetal Materials Spec Sheet, it looks like they offer 316SS, which is non magnetic, although less strong than the 420SS. Bingo.
Prometheus Fusion Perfection 
[…] chassis. The surfacing machine uses a magnetic vise, so the work piece must be magnetic. The sample parts we ordered from prometal are magnetic, however the next parts will be made with a less magnetic […]
[…] Previously we used the 420 Stainless Steel +Bronze, which is magnetic. This time we used the 316 Stainless Steel + Bronze hoping it would be non-magnetic – but unfortunately it is magnetic. […]
[…] I took the opportunity to try welding the 3D printed metal parts I made last year: […]